Cube 2: Sauerbraten is a highly improved version of the open source first person shooter Cube. This sequel again takes away emphasis from highly stylized eye candy (although some of it is still present and clearly visible) and focuses on the ability for the user to create 3D environments with ease. This game works very well on low-end computers,including those without dedicated graphics, so you don't need an expensive gaming rig to enjoy smooth, lag-free gameplay.
Cube 2: Eisenstern
Based on Cube 2: Sauerbraten is a fully finished mod called Cube 2: Eisenstern, which is a single player RPG game.