As far as workout music goes, power metal is consistently uptempo and energetic. Sure, it doesn't have the weak yelling of nu metal (if you want real aggressive vocals, try melodic death metal), but I've found power metal to be great workout music:
Slipknot's Iowa and Volume 3 : The Subliminal Verses are 2 great albums full of rage and energy. They'll make you push the iron as hard as never, or push yourself on your running track, like they do me.
Here's some samples...
And on this one, even the lyrics are kinda very encouraging while doing intense activity, whatever it is.
And appart from Slipknot, I kinda like this one from the last Avenged Sevenfold album, it's called God Hates Us.
awww man where do i start... ill just list some awesome bands
parkway drive attack attack mesuggah asking alexandria suicide silence white chapel enter shikari bring me the horizon norma jean four year strong emmure of mice & men we came as romans the fall of troy
I tend to enjoy entire albums that are pretty non-stop, being a hard music nerd, I find that there are few bands that are able to do such a thing. These are ones I put on repeat:
Snapcase - Progression Through Unlearning Refused - Songs to Fan the Flames of Discontent Botch - We Are the Romans Coalesce - OX Textures - Silhouettes Soilent Green - Inevitable Collapse In The Presence Of Conviction Revocation - Existence Is Futile Gallows - Grey Britain Trap Them - Siezures In Barren Praise Meshuggah - ObZen & Nothing