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.
Blood Makes the Grass Grow Green (BMTG3) is a multi-player, side-scrolling, 1-dimensional strategy game that pits two people against each other in a duel over a stretch of land. Each player is positioned in a bunker at opposite ends of the map and tasked with deploying the right personnel and vehicles to capture FOBs (forward operating bases) and overrun your enemy's bunker. Currently there are 5 different units available. Timing, unit selection, and basic strategy are all necessary for success.
CorsixTH is a clone of Theme Hospital. As computers evolve, we risk losing some classic games. Bullfrog's Theme Hospital, published in 1997, is a classic simulation game, but getting it to run natively on modern operating systems is getting progressively harder. CorsixTH aims to reimplement the game engine of Theme Hospital, and to be able to load the original game data files. This means that you will need a purchased copy of Theme Hospital, or a copy of the demo, in order to use CorsixTH. After most of the original engine has been reimplemented, the project will serve as a base from which extensions to the original game can be made.
Zaz is an arcade action puzzle game where the objective is to get rid of all incoming balls by rearranging their order. Currently it includes 20 different levels and two modes of gameplay. The engine allows for easy custom level creation with unlimited number of paths, different speeds, ball-sizes, and rules. A 3D accelerator is needed for decent gameplay.