Watch the The "Other" Power Chord online guitar lesson by Angus Clark from Hard Rock Survival Guide: Rhythm
Depending on what kind of lineup you are playing with and what kind of riff you're working on, you can use 4ths instead of 5ths as power chords. This is most famously demonstrated by Ritchie Blackmore in most of the legendary tracks by Deep Purple and Rainbow. One thing that's great about writing riffs in 4ths is that you can outline the blues scale box very easily.
Another thing about using 4ths is that you can negotiate certain chord moves that would be pretty awkward if you just use 5ths.
You can also imply some lower chords - like a low Eb.
Things to watch out for - if things are big and muddy - sometimes the 4th on the bottom two strings just clouds things up. 4ths worked great for Blackmore because he used a cleaner tone and a strat - and traditionally he was working in combination with a keyboardist who was playing a root version of the chord. Also, this is not as good a chord for muting, as very often when you mute you mostly hear the bottom string of the power chord, so the muted inversion will just sound like the 5th of the chord. If you're new to using them, you may find it tricky having the root on the top string, but once you get used to it, they are really useful.