I like angry music, or at least stuff with competition-oriented lyrics, and no slow stuff at all. Has to be loud too. I work out so I can be faster in races, and beating people is my motivation, so I like my music to remind me of that conflict while I work out. I'm usually in severe pain while working out so unless I have something really aggressive, I just can't keep going. So, some things I like:
Godsmack - The Enemy (and others)
Metallica - Fuel
Disturbed (several)
Chevelle - Send the pain below
Cake - The Distance
Red Hot Chili Peppers - Can't Stop
Rage against the Machine (several)
Nine Inch Nails (Quake II game instrumental soundtrack)