2+2's Science, Math, Philosophy (SMP) forum is really good:

http://forumserver.twoplustwo.com/forumdisplay.php?f=47

A couple recent threads: - Is my vote mathematically meaningless? - Intelligence, intelligence tests, and psychology - What is space made of? - Professors, Doctors, Lawyers, who's smarter?

Atheism discussions are a particular favorite (which should generally indicate it's the type of forum you're looking for).