This is from 1999, but it's no less interesting today. The winner of the main competition was a bot with a variety of strategies that used whichever strategy was winning, including all possible variants of those strategies based on "but you know I'm going to do that, so you'll do the thing that beats that" logic. Naturally, it was called Iocaine Powder.

There was also a side competition of bots that cheated. My favorite is the one that forked twice, returned all three possible moves, and exited the process if it lost.