I used to use iSquint all the time, but then I upgraded to VisualHub, made by the same people. It is a much more capable tool and doesn't cost very much considering what you get. Of course, iSquint is free, but it doesn't handle all file formats that VisualHub can handle.

I found that both iSquint and VisualHub are problematic if your machine is too busy. They both use applescript and if the system is too busy the script will time out and report a failure, after which nothing gets converted. But they work fine when the system is not completely CPU bound (busy).

isquint totally rocks.. :) I just got it to convert my avi's to mp4's for my ipod :) :)