Haven't seen it mentioned in the comments yet, but here's another that's a bit more hardcore: http://robocode.sourceforge.net/

You code the AI for your tank then let it loose in an arena with other tanks. You have to handle path-finding, scanning, range-finding, target identification, hit/miss analysis.

Some of the winning bots are amazingly competent with their scan-and-move algorithms. You code in Java or some flavor of .Net.