The sites where almost everything "known" known about Star Wars (which isn't all that much, and is still somewhat up in the air even AFTER the teaser has confirmed so much) are
Badass Digest
Indie Revolver
Making Star Wars
There's been stray scoops & such from other sites, but those seem to be the main ones. The only real "what the hell" out-of-nowhere messageboard scoop that caught any sort of traction was an imdb forum post WAAAAAY back, from a guy calling himself mizzlewump, who posted a bullet-pointed synopsis of what he said was the second draft of the screenplay, which obviously would have undergone some changes between now and then. People had been writing it off as bullshit, until the three sites above started also posting stuff that sounded linked to those bullet-points, and now it's DEFINITELY worth looking at as most of the rumors from those three were either outright verified or given some extra weight thanks to the teaser.
If you'd rather listen to the rumors as they unfolded rather than read them, I've done two half-hour podcasts as part of the Rumor Control series for
Full of Sith. You can find them on their site.