Most of the all time ones have already been mentioned, but here are a few good ones, IMO:
the Knack - My Sharona Lenny Kravitz - Are You Gonna Go My Way David Bowie - Rebel Rebel 20th Century Boy - T.Rex Alien Ant Farm - Smooth Criminal Iggy Pop - Lust For Life
Recycling a few already mentioned plus a couple of others.
in no particular order:
Roy Orbison - Pretty Woman
Rage - Killing in the Name of (or pretty much any RATM song - Morello is a riff machine using the traditional riff definition)
Soundgarden - Outshined
Tom Petty - Running Down a Dream
Nirvana - Smells Like Teen Spirit - hate all you want, but this is an insanely popular riff.
Steve Miller Band - The Joker
311 - Homebrew - most people won't know this but listen to it. The opening is a riff and is one of my favorite to pick up and play on my guitar. Simple, repetitive, easily memorable.
Living Color - Cult of Personality - sad this hasn't been mentioned already. HUGE riff!!!!
Lindsey Buckingham - World Turning - but my favorite is Monte Montegomery's version. Skip to 1:23 to ignore the noodling in the beginning. You won't hear the riff if if tune out during the first 1:22.