This is one of the most popular Doom source ports. It is available for DOS, Windows, Linux and Mac OS X, and should build on any POSIX platform which supports SDL. It enhances Doom with things like TCP/IP networking, OpenGL renderer, Heretic support and much more.
Aleph One is the open source continuation of Bungie's Marathon 2 FPS game engine. Aleph One plays Marathon, Marathon 2, Marathon Infinity, and 3rd-party content on a wide array of platforms, with (optional) OpenGL rendering, Internet play, Lua scripting, and more.
Deal or No Deal clone. Select one of twenty-six cases, each with random amounts of money inside ranging from $0.01 to $1,000,000. Then open the rest of the cases, individually, knocking off values. The banker offers amounts of money for your case.
A rewrite of the game engine for Doukoutsu Monogatari (Cave Story) using SDL. The original data files are required to play.