Spin 4
A new version, SpinFour, is available for iPhone and iPod Touch on the iTunes App Store!
Spin 4 is variant-gravity connect four! Huh? It's simple: every turn you can either drop a piece or spin the board. A spin will make the pieces fall into new places. Play right now via Java Webstart.
Connect four has always captivated me; being of my own devising, I am particularly fond of this variation. The game is written entirely in Java using Swing for the UI and rendering. Taking a rather reductionist view of the implementation, the game is naught more than a flashy wrapper around a set of simple 2-D array manipulation functions. The devious AI uses a simple minimax algorithm with alpha-beta pruning to increase the move depth to which it can reasonably search. Grab the source code, available under the GPL license, and find out for yourself.