Great, great thread. Soooo many good answers so far, but I'll throw in a few that I especially like:
Billy Talent - Devil in a Midnight Mass
I think this band is pretty underrated, and this riff is an awesome kick start to their latest album.
Andrew W.K. - She is Beautiful
This song starts out with a stupid starter riff, but once it gets going, it really gets going. Ultimate pump-up riff.
Cake - The Distance
Undeniably cool riff.
Every Time I Die - Apocalypse Every Now and Then
Heavy stuff.
He Is Legend - The Seduction
I've always liked this riff, even if I've kind of grown out of the band.
Franz Ferdinand - Take Me Out
Extremely overplayed, but it retains its awesomeness. The build up to the main riff is the best part of the song.
Jimmy Eat World - Authority Song
Someone already said Get It Faster, and that would've been my first choice from this CD, but Authority Song comes in a close second.
Jimmy Eat World - Futures
Oops, same band. I love a good riff to kick off a CD, and that's exactly what this song gives.
Lovedrug - Pandamoranda
Uncharacteristically heavy riff for this band.
My Chemical Romance - Thank You For The Venom
Suck it. Equally awesome solo to boot.
NOFX - The Separation of Church and Skate
Pretty sweet. Fun to play as well.
Rage Against the Machine - Testify
The Strokes - Reptilia
System of a Down - BYOB
Ted Leo & The Pharmacists - The Angels' Share
Tenacious D - Keilbasa