When I first saw Slayer in 2007, the opening band was Unearth and I wouldn't necessarily say there was booing, but just a bunch of intentional indifference. Slayer has a good track record of late for having kickass bands open for them.
07/22 - Melt-Banana
07/26 - Forever Came Calling & Primitive Man
07/30 - Warped (?)
08/05 - Knockout Kid
08/07 - Fireworks (?)