Clapton's (with Cream) Sunshine of Your Love is a pretty cool riff. Easy riffs, hmm. . .I'll rattle off a few of my other favorties. Round n Round by Ratt, Bang Your Head (Metal Health) by Quiet Riot, Lonely is the Night by Billy Squier, Back in Black by AC/DC, and Righteous by Eric Johnson. Speaking of Eric Johnson, try Cliffs of Dover-the three solos and some of the high-speed runs are a bit ridiculous, but the main riffs are fairly easy. If you noticed, most of these are off of Guitar Hero, that's where I learned a lot of the songs I play. Some really cool acoustic guitar riffs are the intro to Jessica by the Allman Brothers (the electric part is really cool and easy too), and Pinball Wizard by The Who (the strumming is a bit tough on this one). Good luck!
Edit: By the way, music for every one of these songs can be found here on Chordie.
"A steering wheel don't mean you can drive, a warm body don't mean I'm alive"
Switchfoot