Kinda wacky, but the XNA team has had four teams working for four days at GDC on a game. 4 games built from scratch in only 96 hours. Not bad. Not bad at all. You can download the 4 games here (Anyone with XNA Game Studio Express (also free) can play the games on a Windows PC) and read the press release about it here.
Edit: John from GSB posted photos of the games on his Flickr site.
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