ClanLib is a cross-platform game SDK currently supporting Windows and Linux. It provides easy-to-use interfaces for 2D graphics, input, sound, resources, networking, GUI, OpenGL, and more. The goal is to provide a game SDK that does all the dirty work that all modern games need to implement anyway, thus making the game developer concentrate on the gameplay instead of reinventing the wheel over and over again.
console othello is an Othello-style board game running on a text console. The AI is not brilliant and the game is simple, but several improvements are planned, including support for a web version and network play. The source should (theoretically) compile on any platform supporting printf() and fgets(): Linux, Solaris, Windows, BeOS, etc.
Corewars is a game which simulates a virtual machine with a number of programs, where each program tries to crash the others. The program that lasts the longest time wins. A number of sample programs are provided and new programs can be written by the player; Corewars supports both Redcode and its own language.
Crossfire is a multi-player graphical arcade and adventure game made for X. It has certain flavours from other games, especially Gauntlet and the rogue-like games (Nethack, Moria, Angband, and Ragnarok). Any number of players can move around in their own window, finding and using items and battling monsters. They can choose to cooperate or compete in the same world.