from www.oolite.org:

Oolite is a space sim game, inspired by Elite, powered by Objective-C and OpenGL, and designed as a small game that is easy for users to pick up, modify and expand upon. Almost every aspect of the game can be changed by using simple, free graphics packages and text editors.

Originally written for Mac OS X, Oolite is available for Mac OS X (10.5 and higher), Windows XP or later, and Linux. It requires at least a 1 GHz processor, 1 GiB of memory, and a graphics card capable of accelerating OpenGL graphics. The game and source code are offered for free under the GNU General Public License.