A few years ago I wrote a blackjack game for iPods. Since the iPod can't execute third party programs (without alternative firmware), However, they can display interlinked text documents, so I wrote a program in c to generate a document for each possible state of the game. Blackjack made this possible, since there are only 5 or 6 states in any game.

I don't think I ever actually used it except for testing, it was more of a "just because I can" project. To my amazement some people actually used it for fun. I think it got into MAKE blog and a few other cool places.