Cube 2 Sauerbraten is the second iteration of the Cube game engine. Cube is an open source competitive shoot-em-up fashioned after classic first-person-shooters like Quake. The graphics engine is built with OpenGL, and it is above par for free video games both in terms of aesthetics and performance.
Gameplay in Cube 2 Sauerbraten mainly follows the traditional death-match paradigm. Players spawn, collect weapons, and traverse the map trying to frag one another until a time limit or a frag count is reached. A variety of game modes are available for both free-for-all and team games.
Cube 2 Sauerbraten comes with a map editor that's easy to use, so players are not tethered to the default maps. Other things can be customized through an in-game command prompt, and savvy programmers can customize absolutely anything else easily, since the source code is open to survey and change.