Dave Gnukem is a 2D scrolling platform game, similar to Duke Nukem 1. It includes a level editor. It runs on Linux and Windows, and uses the SDL library.
Battlemech is a game in which you steer a mech from a top-down view in various arenas. A lot of weapons and powerups are available. It is meant to be played on a LAN, but you can play against bots, too (though the bots don't score). It is based on an advanced version of the Quake engine and written in QuakeC.