Pompeii worm (photo credit: Wikimedia commons). After water bears (which are apparently pretty common - I didn't know, but you can find them in an average pond!), Pompeii worms are the most temperature resistant complex organisms that exist (capable of living in 80C water, and sustaining huge temperature differences between their head and their tail). They basically live in the gates of hell.
Also: not extremophiles, but lots of neat pictures here of interesting new species. I especially like the Dracula minnow and the psychodelic frogfish.