[gamegoblin]

I came here to mention Robocode. I played the hell out of that back in the day. I am currently working on an indie game (RTS) that includes AI in singleplayer mode, and also allows you to program your own AIs by making a class that inherits from my abstract AI class which gives you access to an AI API. With Robocode there is a pretty high barrier to entry, but my goal is to have the game be entirely playable and a fun game, which then entices the user to go mess with the AI code.