PQV works with most or all AVI or MOV files (motion JPEG videos) generated by digital cameras. I've heard that PQV isn't 'VGA-aware', but at 240x320 QVGA resolution it works great for me. Plays stuff from my wife's Pentax, my brother's Canon, and my sister-in-law's Nikon, with sound, fullscreen and rotated 90°. Still my favourite image viewer.
Other than that, I've read that QuickTime Pro (around $50) will allow conversions to MPEG. And of course
TMPGEncoder or
WMEncoder work fine too, but only with AVI, not MOV types.
Another option is the multi-format player for PPC called
PictPocketCinema. I found that it played some, but not all Quicktime types, and did some other types of files as well. Not worth the cash, in my opinion.