http://www.bautforum.com is pretty good. And
http://www.physicsforums.com I don't read consistently but if there's something specific I often search there and find good threads.
The science forum on JREF is great (
http://forums.randi.org/forumdisplay.php?f=5), there's some actual cosmologists and climate scientists and such that post there, I find their threads on cosmology and climate change awesome, the discussions usually stay very focused and even when heated still have content.