BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
cave9 is a gravity cave-exploration game. It's a first person 3D version of the classic SF-Cave game. Your ship travels into a procedurally-generated cave at constant forward speed, and you have to control two diagonal thrusts to keep it from hitting the walls. As the game progress, the cave will twist more and become more tight.