Another few cool tunings:
like the drop B one from BLOODMOUTAIN's original post but you also drop the A to G so it ends up: BGDGBE, which is the tuning for Tool's Parabola.
Speaking of Silverchair, ever tried Emotion Sickness Mr. Ed (I think its) something like: Db-Ab-Db-Ab-Db-Db
Straight Drop A you can get Muse's Citizen Erased: AADGBE (Yes I know it's originally on a 7 string but you can get away with it like that)
and finally I used to use a really simple open chord tuning: DADADD