HandBrake is an open-source cross platform video transcoder (and DVD ripper)
The command line interface allows you to come up with some powerful scripts to tame your video files. There's some useful documentation under Using HandBrakeCLI
This is a reminder to myself on how convert a video file using the xbox360 preset:
/usr/local/bin/HandBrakeCLI --preset "Xbox 360" -i "input.avi" -o "output.avi"
Combine this with OS X Services to right-click on a file and be able to easily convert it.