Some of my favorite song covers, in no particular order:
Killswitch Engage's cover of Dio's "Holy Diver."
Indica's cover of Popeda's "Kersantti Karoliina."
Demon Hunter's cover of Prong's "Snap Your Fingers, Snap Your Neck."
KoRn's cover of Pink Floyd's "Another Brick in the Wall."
Love and Death's cover of Devo's "Whip it."
Of course, there are many more, but these are the first to pop into mind.
How about you guys?