A developer from the Parallel Extensions team, Igor Ostrovsky, recently whipped together a really neat Silverlight game.  It's called RoboZZle, and he calls it a "social puzzle game":

"RoboZZle is an online puzzle game that challenges players to program a robot to pick up all stars on a game board. The game mechanics are simple, yet allow for a wide variety of challenges that call for very different solution approaches." (from the blog entry introducing it.)

The coolest feature of this game is that you win by programming.  And there's a whole community surrounding it, complete with forums, the ability to create your own games, a ranking system, its own blog and continuously updated news, etc.  Check it out.