Aperture Tag - A modification based on Portal 2 that recreates the essence of the game Tag: The Power of Paint, the inspiration for the various gels used in Portal 2. Instead of a portal gun, the player solves puzzles using a mix of these gels that they can spray onto surfaces with a tool they carry.
Coastline to Atmosphere - A mod set in the Half-Life 2 universe, following Gordon Freeman following the events of Half-Life 2.
Dear Esther - An experimental "ghost story" created as a research project at the University of Portsmouth; initially released as a free modification in 2008, a longer commercial version was developed and released in 2012.
Flipside - A side-scrolling platform game that allows the player to "flip" the game world around to its backside, altering the means a player can traverse a level.
Grey - A total conversion horror mod for Half-Life 2: Episode 2. You take the role of Grey, a man who, after having nightmarish hallucinations on a bar's bathroom, wakes up to find that everyone's gone. Grey exits the bar and heads for his home, ignorant of what will happen along its way. The mod features custom models, sprites, enemies and also a custom animation for picking-up items. The mod can be found at Grey.
Jurassic Life - A modification based on the first Jurassic Park film. The game will act as a side story to the movie where the player takes the role of the park's game warden, Robert Muldoon.
Korsakovia - A single-player mod for Valve Corporation's video game Half-Life 2. It was developed by thechineseroom and released September 19, 2009. In Korsakovia, the player travels through the delusions of Christopher, a man suffering from Korsakoff's Syndrome. The player hears the voices of Christopher and a doctor as they try to work out what is wrong with Christopher.
Nightmare House 2 - A horror-themed modification. The mod won ModDB 2010 Mod of the Year awards for Best Original Art, Best Singleplayer Mod and Player's Choice Mod of the Year categories, eventually winning 2nd place at the latter.
Portal: Prelude - An unofficial prequel to Portal, set before the activation of GLaDOS when scientists used to watch over the test chambers and instruct the test subjects.
Pumpkin Night - Halloween themed single-player HL2 mod with an new, indepth and original storyline taking the player from his suburban neighborhood to do battle with the evils of Halloween Land, and ultimately, the Grim Reaper. Created back in 2005.
Research and Development, a "non-combative" modification that has been likened to Portal, in which the player uses the gravity gun and other aspects of the Source engine physics to solve puzzles and progress forward.
Rexaura, a Portal modification, with puzzles designed around manipulating and ricocheting plasma balls.
Riot Act - A mod set in the Half-Life 2 universe, following a member of the civilian resistance on their escape from the Combine prison at Nova Prospekt.
Age of Chivalry - A multiplayer modification. Players fight for either the Mason Order or Agathian Knights in a fictional medieval environment using swords, spears and many other medieval weapons. Version 1.0 was released on 12/01/07. Age of Chivalry received Mod DB Editor's Choice for 2007.
Dino D-Day - The game is set in 1942 during World War II. The premise of the game is that Adolf Hitler has resurrected dinosaurs, and players can battle online choosing to serve either the Allied nations or the Nazis. The game has been described as a new twist on "the overdone World War II FPS ... that has become a running joke in the industry and gaming press".
D.I.P.R.I.P. Warm Up - A Half Life 2 total conversion featuring gun-mounted vehicles in post war environments.
Dystopia - A cyberpunk-themed total conversion, somewhat based on popular role-playing game Shadowrun, that pits Punk mercenaries against Corporate security forces in both the physical world and cyberspace. Last release February 6, 2009. Dystopia received the Player's Choice Honorable Mention from Mod DB in 2006, Best Mod for Half-Life 2 during the Independent Games Festival in 2006, and received the "Mod Of The Year - 3rd place" and "Genre Award - Action" for mods of 2005 by Mod DB.
Fortress Forever - A mod based on Valve's Team Fortress Classic, but has been created entirely from scratch by the Fortress Forever development team. The intended scope of Fortress Forever is to please the adamant Team Fortress Classic fans, while at the same time creating a game enjoyable enough to players new to Team Fortress styled games. The mod was released on September 13, 2007.Fortress Forever received "Best mod of 2007" in PC Gamer magazine.
Fistful of Frags - A first-person shooter set in the Wild West. Take part in frenzy team versus shootouts or play cooperative games against AI managed enemies. Most of the classic black powder guns are available, in single or dual wield setup. The current version of the modification is 3.8c. The modification was released December 20, 2007 and it become available on Steam as a stand alone free game in May 9th, 2014.
Garry's Mod - A "sandbox" mod that allows the player to manipulate objects using the Sourcephysics engine. Allows both single- and multi-player interaction. Version 13 released commercially and version 9 is still available on Steam. Garry's Mod received Mod DB's "Player's Choice Honorable Mention" from Mod DB in 2006, "Mod of the Year" in 2005 and "Genre Award: Puzzle" in 2005.
GoldenEye: Source - A total conversion for Half-Life 2 that aims to recreate the original Nintendo 64 classic GoldenEye 007. It received Mod DB's "Editors' Choice for Reinvention" in 2006, "Third Place, Mod of the Year" in 2006, and "Fourth Place, Top Unreleased Mods" in 2005.
Half-Life 2: Capture the Flag - A simple capture-the-flag mod. Players are divided into two teams, Combine and Rebels, and try to steal the other team's flag from their base and return it to their own for points, with the aid of ability enhancing runes and various weapons.
The Hidden - Puts one player (called The Hidden) as an invisible genetically-modified human against a team of SWAT-like operatives called the Infinitum Research Intercept Squad, (IRIS) with regular human attributes but a selection of weapons to choose. The IRIS team's objective is to hunt down the Hidden, while the Hidden attempts to stalk and kill the IRIS team without being detected. Hidden: Source won Mod DB's "Fourth Place, Mod of the Year" in 2006, "Editors' Choice for Ambience" in 2006, and "Editors' Choice for Multiplayer" in 2005.
Insurgency: Modern Infantry Combat - Is a total conversion mod for Valve Software's Source engine. Insurgency is a multiplayer, tactical first person shooter, and implements elements of realism, in an attempt to increase player immersion and promote teamwork. The game is primarily set in the Iraq war, however some maps are set in Afghanistan, and future updates are planned to expand the setting into a hypothetical conflict in Kosovo and other theatres. Insurgency received the Player's Choice 2007 "Mod of the Year" award from ModDB, as well as the "Best Source Mod of 2007" Gold Award from Steamfriends.
Jailbreak Source - A team based first-person shooter where each team has a jail in their base. When an enemy player is killed, they will respawn inside your jail, where they can either await release by their own team who are fighting to get to the release button, or escape by their own means, through devious and deadly escape routes. Jailbreak Source has received media coverage and reviews from Planet Half-Life, Total PC Gaming Magazine, and ModDB. Alongside Interviews with ModDB and Interlopers.net
Modular Combat is a Half-Life 2 is a role-playing PvP, PvM, PvPvM and coop shooter based in the Half-Life universe. There are over 50 modules that can be upgraded at any level, in any combination. Modules give combatants amazing abilities such as flying, teleporting, spawning minions, and shooting flechettes or energy balls.
Pirates, Vikings and Knights II - A mostly melee-based mod featuring three factions – Pirates, Vikings and Knights – and includes map objectives such as Deathmatch or Capture the Flag.
Plan of Attack - A multiplayer first-person shooter that highlights strategy, teamwork, and out-thinking the enemy. Last official release October 5, 2005. However, the development team has halted development and released the source code of the mod. Currently a new mod team is recruiting to continue development of the mod.
Resistance and Liberation - A WW2 infantry simulation mod for HL2 that realistically portrays the events and actions taken by the 82nd Airborne during their stay in Normandy. RnL features include no HUD, no crosshair and no death messages to create a highly immersed and team play orientated game play with real 3D voice chat that is range limited.
SourceForts - A capture-the-flag mod. Teams use gravity guns to build a fort out of given parts during the build phases, and assault the other team's fort during the combat phases. One can play as a Soldier, Sniper, Engineer, Rocketeer, or Scout in a fast-paced battle for glory. Last release September 1, 2008.
Thirteenth Place, Mod of the Year from Mod DB in 2005
Players' Choice Genre Award: Action from Mod DB in 2006
Suicide Survival is a modification of Half-Life 2 Deathmatch. There are two teams. You either play as an overweight WW2 pilot in the survivor team or as a suicider disguised as a pot plant in the suicider team. The goal for the survivors is to stay alive and to spot the suiciders. They have a couple of throw-able copies of the book: “Gardening for Beginners – by Barbara Bush” as a self-defense weapon. The goal for suiciders is to sneak up close enough to the survivors in order to blow themselves up. Because each map is full of shrubberies its very hard to spot a real suicidal pot plant among the many fake ones. If a shrubbery isn’t moving, you can’t tell if he is being controlled by another player or not.
Synergy - A cooperative mod. Synergy supports official campaigns by Valve, user-made levels, and other third-party mods (MINERVA, City 7, and Riot Act so far). In September 2008, Valve introduced it as one of the first five Source mods being offered on Steam and supported by Steamworks.
WW1 Source - A World War I modification for the Source Engine, it is an entirely multiplayer-based, tactical first-person shooter placing the player in the shoes of either an Allied or Central Powers soldier on the Western Front, approximately between 1915 and 1918. Each side bears three classes- Rifleman, Machine Gunner, and Officer, with each possessing their own abilities and style of combat. The overall goal of each round is to capture all control points and/or deplete the enemy "morale bar". Released on February 21, 2010.
Zombie Panic! Source - A team-based zombie themed mod which pits player-controlled survivors against player-controlled zombies. The gameplay style simulates an outbreak: The zombie team starts with only a few players, but their ranks grow as survivors are defeated and switch to the zombie team. Its first public release was on December 28, 2007, and it received the Mod DB, players' choice, top unreleased mod award for 2007.
Darkness: Source - A fast-paced asymmetric team game based on the Quake 2 mod, Gloom. Two teams, Spiders and Humans, must destroy the other team's spawn points and players to win.
Alchemilla -A psychological horror mod that uses Half-Life 2's foundation to create an FPS set within the Silent Hill universe.
Lost Story: The Last Days of Earth - A sci-fi FPS action and adventure game based on Source Engine and Half-Life universe with an original and unpredictable storyline featuring single gameplay, with heavy focus on story line. The mod can be voted on Steam Greenlight.
Operation Black Mesa (Opposing Force: Source) - A total remake of Half-Life: Opposing Force. Utilizing the Source engine, Tripmine Studios plans to recreate the Black Mesa Research Facility from the ground up, as seen by the protagonist, Adrian Shephard. Release date of this modification is set to Q1 2014.
Codename CURE - Fast paced zombie killing source mod that is fairly new. The developers have made the game playable on all the most popular OS's e.g. Windows, Mac OS and Linux, which is nice because most mods are only available for windows. The concept of the game is to work together with your team of up to 5 players and get through the whole map without dying. Once you die you re-spawn at the beginning of the map with a new pathway open, making each playthrough dynamic. 
The Survival - A survival horror total conversion mod that will feature an episodic storyline set in the early 1990s and a cast of original characters.