There is a website called arimaa.com that runs a very interesting board game. Many people there are actually researchers, studying this game, and we like people to play so we can build a large database of games to analyze.

The managers there set up a small prize given each month to the player that plays the most that month. I even got the prize a couple of times, it was very nice.

Of course it's something very different, but it serves as a success story for prizes inside an Internet community.