Braid, by Jonathan Blow, is slated for a Spring release.
Penny Arcade Adventures: On the Rain-Slick Precipice of Darkness, by Hothead Studios, is slated for a Spring release. Be sure to check out the three gameplay trailers from January, if you haven’t already.
Castle Crashers, by The Behemoth, is slated for a Summer release.
This information was revealed in a Microsoft press release following their keynote. It’s really more of an excuse to post more screenshots. I want to play all of these games very badly.
(Sources: Kotaku, IndieGames.com Blog)