The list is divided into two categories: clientside and serverside mods. These terms do not necessarily indicate whether each mod can be run on multiplayer servers. Clientside mods add content to the game, such as a new storyline, models or sounds. In contrast, serverside mods add functionality.
Cry of Fear – The spiritual successor of Afraid of Monsters, it puts the player in the shoes of Simon, a paraplegic who writes a book for therapy, and in the process faces the inner demons that plague his thoughts. It features a co-op mode. Though initially a mod, it was later released as a free standalone game in 2013 via Steam.
Escape from Woomera - An unfinished point-and-click adventure game, intended to critique the treatment of mandatorily detained asylum seekers in Australia as well as the Australian government's attempt to impose a media blackout on the detention centers.
They Hunger – This is a single-player horror-based mod. It was released by Neil Manke's Black Widow Games in three episodes, the first in 1999, the second in 2000, and the final installment in 2001. All three were at one point bundled with PC Gamer magazine.
USS Darkstar – This mod follows a scientist on a deep-space scientific mission aboard an interstellar spacecraft. When an experiment goes awry, the player has to defend himself against aliens. This mod was featured by the PC Gamer magazine on its demo CD in 2001.
Wanted! – A Wild West-style mod which follows a town Sheriff and his hunt for a bandit. Enemies include rattlesnakes, Native Americans and other outlaws. It contains original voice acting and era-specific weapons, and was created by Maverick Developments and released as a free mod. For some time it was bundled with the retail version of Counter-Strike, along with another mod by Maverick Developments, Absolute Redemption which was a chapter set between Half-Life and Half-Life 2.
Action Half-Life – A team-based multiplayer mod developed as part of the popular series of "Action" based mods, designed to emulate the feel of an action movie. Members of the development team had previously worked with developers of Counter-Strike on the mod Action Quake 2.
Counter-Strike – A team-based, hugely successful, tactical first-person shooter game involving rounds of combat between two teams, counter-terrorists and terrorists. It was originally a mod and later a commercially released game.
Deathmatch Classic, an official Valve mod, remaking Quake deathmatch.
Team Fortress Classic – Originally a mod ported from QuakeWorld that introduced a class-system style of play that allowed for many diverse playing styles. With the introduction of Steam it became a standalone game and as of October 10, 2007 it has a commercially released sequel Team Fortress 2.
Global Warfare – A team play class based mod set in the Middle East with frantic gameplay and new features (e.g. troop transports taking players into battle).
International Online Soccer – This is a first and third-person multiplayer soccer game. 2006 winner in Sports category, Mod DB Player's Choice
Master-Sword – A Roleplaying Game originally released in 1998, it has since gone through several revivals. Master-Sword has often been one of the most popular mods on ModDB and it is one of the few mods to still survive.
Natural Selection – A mod in which two teams (humans and aliens) fight against each other. Its utilizes a mixture of first-person shooter and real-time strategy gameplay. It gained a standalone successor, Natural Selection 2.
The Ship - Set on a 1920s recreational cruise ship. Each player is assigned a quarry, and the object is to murder him or her with nobody watching, while at the same time watching out for, and defending themselves against their own hunter. Later released as a standalone game on the Source engine.
Sven Co-op – A co-op mod in which players fight against computer-controlled enemies. Apart from user contributed missions it is also possible to play co-operatively through the original Half-Life game.
The Half-Life serverside engine could originally only support one mod at a time, because it used to load a DLL which contained the standard engine as well as the mod.
Metamod is a serverside mod that fixed this, allowing multiple mods to be run at once. Many such mods run under Metamod, referred to as Metamod plugins.
The AMX Mod was a plugin running under Metamod that allowed scripts to be written for the servers. It got its name from the filename extension of compiled Pawn files, which stands for Abstract Machine eXecutive. In 2007 the AMX Mod project was merged into the related AMX Mod X, which had more features.