There are loads of classical pieces that work well, but the problem I've had is getting works that aren't quiet as balls. I realize that a lot of music, as released, is much louder than strictly necessary, but when making a playlist it helps if everything is at the same volume.
If you can find good recordings, Antonin Dvorak's New World Symphony is pretty sweeping: http://www.youtube.com/watch?v=yctfXIqugXc. Richard Wagner has lots of awesome shit, too, beyond the obvious Ride of the Valkyries stuff. Siegfried's Funeral March (http://www.youtube.com/watch?v=a53s4jyCqqU) is good, but really only if the battle is going poorly or something really dramatic is happening.
Also, EVERYTHING by Hans Zimmer (Inception was mentioned upthread, but Gladiator, Batman Begins and Dark Knight [with James Newton Howard), Harry Gregson-Williams (Metal Gear Solid, natch). But when shit goes down Verdi's Dies Irae is always a good choice (http://www.youtube.com/watch?v=pW1Uc-grcMs).
Some anime soundtracks are good, too. The Kenshin OVA soundtracks (not sure of composer) are good, as is the Escaflowne soundtrack.