Oolite is a space sim game, written for Mac OS X in Objective C using Cocoa and OpenGL. It is designed as a small game that is easy for users to pick up and expand upon. Almost every aspect of the game can be modified using simple, free graphics packages and text-editors. Oolite now works in Tiger. [Link]

Update: Although the idea behind this game sounded promising, after playing it or should I say trying to play it, this game is basically crap.