ioquake3 (or ioq3 for short) builds upon id Software's Quake 3 source code release. It cleans it up, fixes bugs, and adds features. Its goal is to be the Quake 3 distribution upon which people base their games and projects. Its design goals include optimizing the engine for playing Quake 3: Arena, Team Arena, and all popular mods. This distribution of the engine has been ported to many new platforms. While it doesn't have PunkBuster (and never will), it does provide more security for servers and clients by way of various bugfixes which aren't in id's client.
Bitfighter is a game that feels a little like Asteroids on steroids. It's a multiplayer space-based combat and strategy game, featuring customized ships, multiple game modes, cool retro graphics, and smooth network play. It also includes a full featured level editor, Lua scriptable bots, and other cool features. It has a very active development team, so the game keeps improving.
Penumbra is a first person horror adventure focusing on story, immersion, and puzzle solving. Violence and combat are hardly an option: the player has to use wits to guide Philip through his final test, which makes the series unique in offering a truly dangerous and terrifying experience. Played from a first person perspective and powered by a 3D engine utilizing cutting edge technology, Penumbra is a true advancement to the adventure genre. The unique physics system allows for true environmental interaction: The player can open drawers, pull levers, pick up objects and manipulate those objects in 3D space, all using natural mouse movements. This creates a highly interactive and immersive game world.