I have always loved games that masterba… er, play themselves out. In such games the player sets the initial conditions– perhaps even writes a little code or designs something– and then lets it all hit the fan.
As of late I have been staring at Gratuitous Space Battles. In this one you design a small fleet of space ships, complete with hulls, engines, weapons, shields, and the like, and then set them against waves of enemies. The beauty of the game is no setup will work equally well against every enemy (at least once you are past the first few levels, that is).
This has re-sparked interest in my idea for something I am currently calling Evolution Battle (yes, yes, it is a dumb name). I envision it as SimLife mixed with something similar to Gratuitous Space Battles. Players would create the “life” with its basic attributes and then stick them in the world with other “life” to compete for resources. I think it would be a great project for me since it would involve a few technical challenges I am not sure I have encountered before.
Just an honorable mention for a few of my other favorites: Crazy Machines (a near-clone of The Incredible Machine), the classic Conway’s Game of Life, Robocode, Bloons Tower Defense 3, and Lemmings (sort of).