RTanque is a game for Ruby programmers. Players program the brain of a tank and then send that tank to battle against other players' tank brain programs. Implementing a tank brain may sound like a difficult or time consuming task, but the API is pretty straightforward and easy to use. I put together a simple bot in less than 30 minutes that provides a decent challenge against the keyboard controlled bot.
Note: I recommend using Ruby 1.9.3 with RTanque. It depends on Gosu which isn't compatible with Ruby 2.0 on my system.