Robocode

Robocode is a free game provided by IBM. It is designed to help people learn to program in java and enjoy the experience. Be warned, this is an addictive game. It is very easy to start - you can have your own bot within minutes - but it may take you months to perfect it.

The idea is that you write the software that controls a little tank which shoots at other tanks on the screen. The rules are very simple: you move, you shoot, you scan, you hit the walls if you aren't careful.