I thought I'd post links to some MMA-related music, for the enjoyment of all. Feel free to add your own.
I don't know if I need to post a disclaimer on this site, but there's some "harsh language" in some of these songs (in more than one language), so don't be a doofus and crank up the volume while you're in class or make your 5-yr-old sister listen to them over and over.
The PRIDE FC theme. New UFC fans might know it as Cro Cop's entrance music. Old Pride fans might get a little misty-eyed. *sniff*
The UFC theme song. "Face the Pain", by Stemm.
B.J. Penn's entrance music. "Hawai'i 78", by Israel "Iz" Kamakawiwo'ole.
B.J. Penn's UFC 63 intro. Uses the opening bars of "Hawai'i 78" by Iz, and then segues into "Crazy" by Gnarls Barkley.
Music no UFC fighter wants to hear. 
"Enae Volare Mezzo", by ERA.
Chuck Liddell's entrance music. "Intro" by DMX, from the album It's Dark and Hell is Hot.
Georges St. Pierre's entrance music. "Samurai", by Shurik'n.
Matt Hughes' entrance music. "A Country Boy Can Survive", by Hank Williams, Jr.
Quinton Jackson's theme song. "Rampage", by PAK.
Sean Sherk's entrance music. Starts with the "Theme from 'Jaws'" by John Williams, then segues to "Wait" by Earshot. Thanks to MrLehman for the link.
Misc. Links:
A listing of UFC fighter entrance music. Thanks to Rush for the link.