1. Doomsday Engine
A portable game engine for classic first person shooters such as DOOM, Heretic and Hexen. Lets you enjoy the original games using modern technology, including high-resolution OpenGL graphics, 3D models, and dynamic lighting effects.
2. AMX Mod X
AMX Mod X is a Metamod plugin which allows you to write modifications for Half-Life with Small scripting language. It provides in-game administration tools, game statistics, server management and gameplay modificators. It is based on the original AMX Mod
3. Digital Paint: Paintball 2
Paintball2 is a fast-paced first-person game with capture the flag, elimination, siege, and deathmatch (free-for-all) styles of gameplay. This project focuses on enhancing the Quake2-based engine it uses.
4. Nexuiz
Nexuiz is a multiplayer first-person shooter. It is distributed under the GNU General Public License (GPLv2). Version 1.0 was released on May 31, 2005. The current version is, 2.5.2.
5. Tremulous
Tremulous is a free, open source game that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. Other structures provide automated base defense (to some degree), healing functions and much more...