I used PSP Video 9, Xvid4PSP and SUPER in the past. PSP Video 9 is not the best one out there, but does the job properly, Xvid4PSP was the best MP4 converter (worked for any MP4 device, including cellphones and iPods) by far I've ever used, but version 5.0 introduced a headache inducing interface and 4.x isn't available anymore. Now I mainly use SUPER. It can convert anything to anything, but has some weird limitations. Still, it's fast and works good.