GamesRadar let us know that they’re passing out 500 Diablo 3 beta keys, 50 a day for the next 10 days, to anyone who becomes a fan of their Facebook page. The contest info page is here, and you can befriend them on Facebook here.
They’ve got 56k friends already, so it sounds like your odds are less than 1/100, but you never know. Good luck.
Update: Gosu Gamers is giving away 150 keys through five different contests, all of which are better than Facebook liking. The entry deadline for all of them is March 4th.
Update #2: Vox Games is giving away an unspecified number of keys, also via their Facebook page. Like them and hope to win all week.
Update #3 PikiGeek are on day two of their giveaway.