Good Covers (that haven't already been mentioned)
Stone Cold Crazy-- Metallica****(Queen)
Hallowed by Thy Name-- Cradle of Filth (Iron Maiden)
The Hunt-- Sepultura (New Model Army) One of my fave songs of all time, methinks.
Across the Universe-- Fiona Apple (Beatles, already mentioned but worthy of another mention)
War Pigs-- Faith No More (Black Sabbath, see above)
Mrs. Robinson-- Pennywise (Simon & Garfunkel?)
Close to Me-- Get Up Kids (The Cure)
Molly's Lips-- Nirvana (The Vaselines)
Super Marios Bros Theme Song-- Mr. Bungle (Nintendo?)
War Ensemble-- Bigwig? (Slayer)
Harvester of Sorrow-- Link80 (Metallica)
This one makes me blush to admit

o), but I like Limp Bizkit's remake of George Micheal's "Faith" better than the original. But then again, I'm not too fond on George Micheals in the first place, so the bar wasn't too high; LB just made a shitty song less shitty.
And a mentionable cover, although not great, is Melt Banana's version of "We Will Rock You" by Queen.
**Afterthought:**Actually most of Metallica's covers on the Garage Inc. Cd aren't too bad.