Disturbed are pretty good for this; I won't likely be using music when I train until I set up a private gym (=stereo=no headphones=relatively low distraction), but they're a favorite of several of my friends (oh, and of course Matt Kroczaleski - and come on, if they're good enough for one of the strongest men of our time, I'm pretty sure they'll work for most beings of lesser strength and dedication).
Amon Amarth are decent, particularly their most recent album (and then particularly the title track...possibly even only the title track).
Judas Priest's heavier side is also pretty good. Painkiller, for example.
Depending on how tolerant you are of having to constantly change song, Opeth can work quite well. Most of their songs open heavy or at least dynamic and then lapse out around the 1, 2, or 3 minute mark. Ghost Reveries is a pretty album there, though if you're just after a quick boost it's pretty much impossible to top the first minute of Deliverance (from the album of the same name).
Some Emperor can work; same with other black metal bands (I'm told Immortal is a good one here, but I haven't heard any of their stuff in years and can't remember much there).
Some thrash can work, depending on how you listen to it and/or whether or not you can stand it. I don't particularly like enough of the genre to be able to use it, though (...Master of Puppets is not motivational if you discover it after Painkiller.).
I hesitate to even say the word 'DragonForce' but the intro to Disciples of Babylon is pretty effective if you're just after something quick.
I'm told good things about some more obscure stuff like Anaal Nathraak, but I can't bring myself to actually buy it.
Some Maiden. The Trooper can be useful if you imagine it with greater distortion.
For some reason, I've actually set new records pretty effectively while playing really weary stuff like Barry Manilow - Mandy and Dream Theater - Space-Dye Vest in my head. Speaking of DT, most of the stuff on Train of Thought.
Can't think of any others. Frankly, if I'm not in a gym where music from headphones would be drowned out by sleds grating on concrete, bars slamming into racks, plates hitting the floor, and people grunting, screaming, roaring, and yelling with exertion (not to mention cuing each other and me on every heavy rep), motivational music for playing on anything other than my mental stereo is a secondary concern until I can find (or establish) such a gym.
