The Half-Life series spawned countless mods, among which many that became highly popular in the Half-Life gaming community. Some original mods, such as Counter-Strike, Team Fortress (originally a Quake mod ported to the GoldSrc engine at Valve's request), Day of Defeat, Ricochet, The Ship and Alien Swarm (originally a mod for Unreal Tournament 2004) have been "adopted" by Valve and made into commercial games. However some mods such as Garry's Mod having been turned into commercial games remain independent. Some mods are also based on cut content, featured in the released files for Half-Life, and in the Half-Life 2 leak.
Some of these mods include minor or major alterations to the original game, others consist in a total conversion, i.e. made from scratch, only using the existing engine, models, sounds, textures, etc. The non-commercial mods can be downloaded for free, as they are not licensing the engine.
 Notable mods
Action Half-Life simulate action movies, especially those directed by John Woo.
Afraid of Monsters is a survival horror single-player modification for Half-Life, made by Team Psykskallar, the creators of Cry of Fear. It is played from the point of view of David Leatherhoff - a man addicted to drugs, experiencing fearsome illusions, and seeking help.
Counter-Strike (originally marketed as Half-Life: Counter-Strike) is a multiplayer first-person shooter initially created by Minh Le and Jess Cliffe as a mod for Half-Life but eventually turned into a retail game developed by Valve.
 Azure Sheep
Azure Sheep is a single-player modification for Half-Life, in which the player is a member of the Black Mesa Security Force. When the chaos caused by the Resonance Cascade starts, he goes on a mission to save his colleague Kate, trapped in the complex.
In Black Ops, the player slips into the role of agent Declan Walker, member of the Black Operations, a special-purpose force for covered operations, created by the government, sent on a mission to eliminate a scientist and arm the Mark IV Thermonuclear Device.
In Cat-Life the player enters the mind and body of an oridinary house cat that finds himself in the Black Mesa facility during the Resonance Cascade.
 Cry of Fear
Cry of Fear is a single-player, psychological horror first person shooter made by Team Psykskallar, creators of Afraid of Monsters. Originally a modification of Half-Life, it is now a standalone product available on Steam.
The story begins with Simon Henriksson, a 19 year old, waking up in an unknown alley shortly after being hit by a car. Much of his time is spent exploring and fighting off crazed, deformed humanoid creatures. He also receives flashbacks telling the story of what happened after the accident, varying from the police response to the resulting hospital stay.
Day of Defeat is a team-based, multi-player, World War II, first-person shooter computer game of the European Theater of World War II. It takes place around the year of 1944. Originally a Half-Life third party modification, it was turned into retail games developed by Valve.
Deliverance is a single-player modification that continues the story of Gordon Freeman shortly after the end of Half-Life.
 Drug Barons
Drug Barons is a singleplayer level pack for Half-Life by Maverick Developments. It was released in March 1999. It takes place after the events of Half-Life. Gordon Freeman accepts the G-Man's offer, and joins the government department that he works for. Back on Earth, the G-Man gives Gordon a mission, and then sends him on his way. The mission is to destroy a drug factory located in a remote South American location.
Half-Life: Visitors is a single-player modification for Half-Life, played from the point of view of Dr. Alan Green, a young lawyer, who was sent to the Black Mesa desert as a safety evaluator to carry ot a safety report of Black Mesa.
Half-Life: Induction is a prequel to Half-Life, in which the player assumes the role of Gordon Freeman.
Natural Selection is a multiplayer Half-Life modification that combines first-person combat and real-time strategy. It features "marines versus aliens" teamplay, where one marine can become a commander, who plays from a top-down perspective and leads live players, while aliens can evolve and change lifeforms to run on walls, fly or devour marines whole. On 31 October 2012, a standalone sequel to Natural Selection was released.
PARANOIA is a total conversion single-player modification for Half-Life, player from the point of view of a Russian officer.
Peaces Like Us is a single-player modification, set in a world where mankind is allied to the aliens of Xen.
In Point of View, a single-player modification created by the author of Azure Sheep, the player enters the body of a Alien Slave named Xonxt. The game starts as humans invade Xen and capture the XTBS, which is an important piece of Xen technology. The game follows this Vortigaunt's journey as it attempts to recapture the device. It features new weapons, as well as both new and old areas of Black Mesa facility and Xen.
Redemption is a single-player modification for Half-Life, developed by Maverick Developments, released in October 1999, while Absolute Redemption is a retail game, released in November 2000. The mod was included in the original retail version of Counter-Strike.
Redemption takes place after the events in Half-Life. The game assumes that Gordon Freeman accepted the G-Man's offer. Now an agent, he is given a task to retrieve three Telnorps and return them back to Xen.
Ricochet was originally a multiplayer mod of Half-Life, but now is a retail game developed by Valve, notable for its apparent simplicity, inspired by arcade and platform games in both concept and design. Players jump between neon-lit platforms suspended above an abyss, and shoot disks at one another, trying to push the opponent off the platform and into the depths.
Scientist Slaughterhouse is a single-player modification of Half-Life that allows the player to murder scientists in many different ways, without any consequences.
 Sven Co-op
Sven Co-op is a multiplayer cooperative modification for Half-Life, in which players must work together against AI-controlled enemies and solve puzzles as a team. It is one of the oldest Half-Life mods to be still in development.
 Sweet Half-Life
Sweet Half-Life is a single-player mod for Half-Life, set at a similar time to the original game and following a similar storyline. It is played through the eyes of Gordon McGinley, and follows his plight to escape from the crumbling Black Mesa Research Facility.
Team Fortress is a class- and team-based multiplayer online first-person shooter, originally released for Windows on April 1, 1999 as a mod for Half-Life but later turned intro a retail game developed by Valve.
Originally a multiplayer mod for Half-Life, it was later turned into a retail game.
 The Specialists
The Specialists is a multiplayer modification for Half-Life. Originally made to be a team-play game, The Specialists evolved into a death-match game with action movie realism.
 They Hunger
They Hunger is a single-player horror mod for Half-Life developed by Black Widow Games. It focuses on a small American town in the 1950s, where the player is an author that has moved to the town to get inspiration for his new book.
 TWHL Tower
TWHL Tower is a single-player mod for Half-Life, collaboratively created by members of The Whole Half-Life community.
 USS Darkstar
USS Darkstar is a Half-Life mod created by Neil Manke and his team, largely based in Canada. The mod follows Gordon Freeman and several other scientists on board USS Darkstar in the year 2066, a large exploratory starship.
Wanted! is a singleplayer and multiplayer total conversion for Half-Life, played from the point of view of Sheriff Rogan Black.
1187, a Half-Life 2 mod, takes place in Amstelveen, Netherlands during the Portal Storms.
Age of Chivalry is a total conversion multiplayer modification for Half-Life 2 made by Team Chivalry.
 Aperture Tag: The Paint Gun Testing Initiative
Aperture Tag: The Paint Gun Testing Initiative is a Portal 2 modification based on the game Tag: The Power of Paint. It features the Aperture Science Paint Gun Device - a tool capable of spreading out an unlimited amount of Repulsion or Propulsion Gel - instead of the regular Portal Gun.
Awakening is a Half-Life 2 mod that continues the story of Half-Life: Opposing Force, and is set before the sequel.
 Black Mesa
Black Mesa is both a third-party total conversion mod for Half-Life 2 and remake of the original Half-Life. The ultimate goal of the mod is to recreate the original Half-Life from the ground up, using the advanced capabilities of the Source engine to create a new and more engrossing in-game world with more varied, complex environments and more challenging, realistic gameplay.
Black Mesa: Uplink is a third-party remake of the original Half-Life: Uplink, one of the demo versions of Half-Life.
 The Citizen
The Citizen is a series of single-player mods for Half-Life 2, set in a storyline parallel to the original game. The player takes the role of an ordinary man, pushed over the edge by the oppressive regime controlling his life. His home trashed, his family dead, The Citizen leaves his apartment for the last time. He seeks new friends, and a way to help the human resistance to smash the iron grip of the Combine. What follows is a winding tale of betrayal, determination and grief. Many obstacles lie in his path which require a sleek combination of agility, stealth, intelligence, and sheer violence to overcome.
Coastline to Atmosphere is a single-player mod for Half-Life 2, released in 2006, that continues the story of the game.
 Combine Destiny
Combine Destiny is a Half-Life 2 mod played from the point of view of a Combine Soldier.
DayHard is a single-player Half-Life 2 mod, set before the events of that game, in which the player assumes the role of Gordon's brother - Morgan Freeman.
Dear Esther is a first person art video game, released in June 2008 as a free Source mod, and later redeveloped for a commercial release in February 2012. The game does not follow traditional video game conventions, as it involves minimal interaction from the player and does not require choices to be made nor tasks to be completed. It instead places focus on its story, which is told through a fragmented, epistolary narrative as the player explores an unnamed island in the Hebrides.
Dino D-Day is a multiplayer team-based total conversion modification on the Source engine, in which the players can battle online choosing to serve either the Allied nations or the Nazis, with both sides aided by resurrected dinosaurs.
D.I.P.R.I.P. is a Half-Life 2 multiplayer total conversion modification, based on vehicle combat and set in various post-war environments.
Dystopia is a multiplayer cyberpunk-themed mod for Half-Life 2.
 FakeFactory's Cinematic Mod
FakeFactory's Cinematic Mod is a modification that aims to improve Half-Life 2 and its episodes, and push the Source engine capabilities as far as possible.
HD Alyx based on Brazilian model Adriana Lima in Version 8 of FakeFactory's Cinematic Mod.
The Overwatch Nexus in Version 10.
 Garry's Mod
Garry's Mod is a sandbox modification for Half-Life 2. It allows the player to manipulate objects and experiment with the physics of the Source engine, often with interesting results due to the power of the engine. Being a sandbox type game there is no goal as such or a way to win, rather users are able to modify the game and play the mod however they wish. Many different gamemodes have emerged with many different objectives and styles. Some even choose to remain in the sandbox mode to construct contraptions or scenery with the props and maps from most of Valve's games.
The current version of Garry's Mod is version 13, which still receives regular updates via Steam Valve no longer categorizes Garry's Mod 13 as a modification, but rather as a full game, due to its ubiquity and popularity.
Eternal Silence is a cinematic multiplayer Source engine modification that takes place in the outer space.
GoldenEye: Source is a total conversion modification of Half-Life 2, and a remastered verison of the original GoldenEye64.
Guard Duty is an upcoming remake of Half-Life: Blue Shift on the Source engine.
Half-Life 2 Capture the Flag is a multiplayer modification of Half-Life 2 that centers around the title game mode.
Half-Life 2: Riot Act is a single-player modification for Half-Life 2. It is played from the point of view of Jamil Lee, a young member of the civilian Resistance, taken by the Combine and currently imprisoned in Nova Prospekt.
Half-Life 2: Short Stories: Human Error is a Half-Life 2: Episode One modification, in which the player takes the role of a Civil Protection officer called Anders.
 Lambda Wars
Lambda Wars, previously known as Half-Life 2: Wars, is a multiplayer Source engine modification that brings the war between mankind and the Combine to a real-time strategy setting.
MINERVA is an episodic single-player modification for Half-Life 2 created by Adam Foster and is a follow-up to his previous mod for Half-Life, Someplace Else.
 Missing Information
Missing Information, is a single-player modification based on the cut content from the Half-Life 2 leak.
 Mistake of Pythagoras
Mistake of Pythagoras is a single-player modification for Half-Life 2, developed by the creator of Sweet Half-Life. The player takes the role of Gordon Freeman in a parallel world.
Nuclear Dawn is a post-apocalyptic first-person shooter real-time strategy hybrid video game, originally announced in February 2006 as an amateur mod for the Source engine, but in April 2009 was revealed to have become a commercial project, still using Source. It was released on September 26, 2011.
 Obsidian Conflict
Obsidian Conflict is a community-based cooperative mod for Half-Life 2 and its episodes.
 Operation Black Mesa
Operation Black Mesa is an upcoming modification for Half-Life 2 and a complete Source conversion of Half-Life: Opposing Force.
 Portal: Prelude
Portal: Prelude is a modification for Portal which aims to extend the Portal storyline.
 Portal Stories: Mel
Explores the story of Mel, protagonist cut from Portal 2.
 Portal: The Flash Version MapPack
Research and Development is a puzzle-centric single-player mod for Half-Life 2: Episode Two.
SMOD is single-player modification which adds new effects, weapons, and NPCs to Half-Life 2.
SourceForts is a Half Life 2: Deathmatch team-based mod, in which player build bases and other structures with props in a "build phase", and then fight and try to capture the enemy's flag in a "combat phase".
Synergy is a co-operative multiplayer modification for Half-Life 2, Lost Coast, Episode One, Episode Two, and SiN Episodes: Emergence. It also includes fan-made cooperative maps, as well as support for the mods MINERVA, Half-Life 2: Riot Act and City 7: Toronto Conflict. It is available for download on Steam.
 Thinking with Time Machine
Thinking with Time Machine is a Portal 2 modification that introduces new time-based mechanics in addition to the standard Portal Gun.
 Weekday Warrior
Weekday Warrior is a mod for Half-Life 2. Set inside a modern corporate office environment, the player takes on the persona of Doug Smith, a very bored man with the untapped potential for greatness.
Zombie Master is a multiplayer Half-Life 2 modification, centred around a zombie-apocalypse, which throws all but one player in the shoes of human survivors.
 Zombie Panic! Source
Zombie Panic! Source is a multiplayer Half-Life 2 modification that pays homage to the popular Zombie genre of movies and games.
- The Valve Developer Community - official knowledge base for developers
- Mod DB - Mod database
- Planet Phillip - mods for both GoldSrc and Source engines, and some sneak peeks on unreleased mods
- Interlopers.net - Half-Life 2 News & Tutorials
- Half-Life Downloads on FilePlanet
- Half-Life 2 mods
- FPSBANANA - The FPS Customization Community