Rise Against (melodic hardcore/pop punk)
The Agonist's singer is a vegan and super duper animal rights activist, though I don't think a lot of it comes across in her lyrics.
Machine Head has a couple of songs off The Blackening aimed at war, religion, etc. (specifically "Clenching the Fists of Dissent" and "A Farewell to Arms")
09/19 - Crowbar/Revocation
10/06 - The Black Dahlia Murder
11/30 - Slipknot/Korn
12/27 - Protest the Hero