So I've recently became a pretty big fan of a lot of death metal bands. My workout playlist is gradually being taken over.
Here are some of the bands I currently listen to while lifting:
Amon Amarth (so epic and amazing)
All That Remains
Here's an example: http://www.youtube.com/watch?v=yGywo81G6lk (one of my favorite songs of all time, even my friends that don't like death metal like this one)
Anything with a good melody, good progession, etc. I usually don't like grindcore unless it's done extremely well (ie like Amon Amarth).