These are really good sites and forums for budget travel:

http://www.eurotrip.com/forum (good emphasis on finding cheap fares, plus a lot of other subjects)
http://www.bugeurope.com (the hostel reviews are very helpful, the forum is not too active)
http://www.guideforeurope.com (click on the forum link)

Edit: I just realized you are focusing on US travel, and most of those are Europe-focused.

Oh well. My main trip for US budget travel is to use Priceline judiciously.