IBM alphaWorks' Robocode is sweeping the net.

Basically, the idea is that you write the code for a program which controls a virtual robot. An armed-to-the-teeth robot. Looks like excellent fun. I must give it a go.

See also this article on developing Robocode with Eclipse.

It's not the only game in town, though - see also intergalactics. I'm generally more of a strategy man anyway, so this looks even more interesting, if rather harder.