I learn some Paul McCartney bass lines. Seriously. Learn some of the scale licks paul plays. These are a great addition to your bass vocabulary.
+1 on this. McCartney has a way of moving a bass line that just seems to add something to the music. It moves, but it's not too busy. Even if it doesn't work for your particular song, knowing some McCartney bass lines is a good thing.
We do a Ramones medley, and basically I just thump on the root. But it works for that music. Don't over-complicate music that was meant to be played simple. The punk rockers love that simple stuff. Just play it with passion.
We used to play "Mother" by Danzig
http://www.youtube.com/watch?v=5RPrj0bp0NA&ob=av2e. On that song, the lead and rhythm guitar are hitting power chords and moving around on a pattern. The bass, however, just sits there thumping on the pedal note (we played it in D) while the guitarists are moving around. That adds some interesting tension to the song to hear the bass sitting down there while everything moves around that one bass pedal note. Then, when I joined the guitars towards the chorus and hit the root of their power chords with them, it broke the tension and the song really rocked out. OK, not the most interesting bass line I have ever played, but it served the song. In that case, less was definitely more.
Another good example of that idea is the main bass riff for "Jump" by Van Halen
http://www.youtube.com/watch?v=k8LdRJqjjRM. Michael sits there forever pounding out the C note with Ed on the keyboard hammering out his chords. Again, simple bass line, but effective for the song.
Recently, we added "Last Nite" by The Strokes
http://www.youtube.com/watch?v=f1vvUec71v8 to our set list. I really like that song because it's the bass that plays the "melody" line while the guitars are holding down a steady beat strumming out simple chords. That's just the opposite of most songs we play where I hit the pocket with the drummer and let the guitarists stand out. Don't know if you could force a bass melody line in your music, but it works on Last Nite.
Anyway, I'm signing on to this thread to see what some of the other guys suggest. I'm often in your exact situation and wonder what would work best on bass when the guitars are hammering out on power chords.