A powerchord is a powerchord, once you can do one you can do em all. It's just a matter of sticking them together to create a riff. Is it riffs you're looking for or exercises? A riff that helped me when I started out was For Whom The Bells Tolls by Metallica (back in the day!), it's fairly simple but involves palm muting, rythmic strumming and moving power chords. Also it sounds cool;-)
All depends on what level you're at. If you want to take power chords to the extreme try working out some Death or Nocturnus!