My friend Herodin bought an interesting game recently. It's called Pentago. Here is my Flash version of it.
The goal of the game is to get 5 balls in a row. After dropping a ball the player is forced to rotate the plate the ball was dropped onto by 90 degrees. The direction of rotation can be chosen freely.
It's a two player game, no A.I. or multiplayer features for now. The only clever stuff that IS done is the check if one or both players won to end the game. This is achieved by breaking down the whole board information to a string (which can be used for replay and network play) that is then handed to an algorithm that identifies if the game is over. It can easily be extended for network play.