NERO is a machine learning game in which the player uses real-time neuroevolution to train a team of robotic soldiers for combat. Training takes place in a sandbox, where the player can place obstacles and enemies, and change the fitness function according to the skills that should be learned. After training, players can pit their teams against other teams in combat!

Videos of NERO in action are available here:
http://z.cs.utexas.edu/users/nn/nero/video.php

This link is part of the NERO homepage, where you can download the game to try for yourself:
http://nerogame.org/