I'd had a lockup (old Classic game) in OS X. Next time I accessed the 'open with' dialog I got a long list of options, with multiple versions of older applications appearing (ghost apps).
Safe start fixed it, one of the most basic troubleshooting tips: Gordon's Tech: Mac OS X Update troubleshooting. I held down shift and restarted. Takes a long time (it's the easiest way to run fsck in Tiger). Then restarted again. Problem fixed, drop down list repaired.