Grobots is a real-time strategy programming game, in which programs (in a dialect of Forth) control teams of robots as they forage, reproduce and fight. It runs on Mac OS X, Windows, and (without graphics) Linux.
The most complete programming game I've played. You have to balance between resource gathering, combat, and territorial control, lending a wonderful level of complexity to the programmer. Better documentation and/or a side editor might make the barrier to entry a bit lower, though. More games like this, please!
Best programming game I've played.
This is a very neat project. I spend maybe a couple hours a week tinkering with my bots and examining others. It has good promise for extensibility and porting, and I for one will be watching it for the foreseeable future.