http://www.youtube.com/watch?v=QgrFx0cDWQg&feature=related
http://www.youtube.com/watch?v=OVpszlmUv0I&feature=related
http://www.youtube.com/watch?v=h7xv5ItAqpY&feature=related

http://www.youtube.com/watch?v=aJo_3B5-1uk

Course you could go with the standard Mortal Kombat music.

Some really awesome suggestions here, especially from Exorince (the Rocky music is awesome too).

However this thread would be incomplete without http://www.youtube.com/watch?v=R8D08WuHNTE. You don't even need to work out to it, just listening makes you bigger.

Hearts on Fire from Rocky 4 is a great workout song.