While you may need an old operating system to play it,its still cool to see an evolutionary algorithm put into game AI. This site hosts download for an Alife game that offers its source code called bSerene is a game where monsters learn to play against the player via Artificial Life. The behavior of the monsters is governed by about 50 parameters, which are used as genes in a genetic algorithm. At the end of every arena the monsters that have inflicted the most damage on the players are saved to file and used as the parents of monsters in subsequent games. Monsters who are close to another monster that manages to hit you get a percentage of the credit.
Without having to look at the source code: You can change the appearance of the program by putting in your own pictures for the explosions power-ups and projectiles, or you can design your own arenas by simply modifying some txt-files in the game folder. The source code and executable game is provided for download here.