If you haven't heard of it yet, A Map of the Floating City is live. It is a mystery and trading game written and produced by Thomas Dolby (yes that Thomas Dolby. And if That Thomas Dolby means nothing to you, I don't want to live in the same world as you).
As you play the game you can win prizes, so far, I've picked up a couple of MP3 downloads for free.
If you like casual cooperative games, it's interesting, and with real world prizes on the line, there's more of a reason to play than a virtual reputation.