I just started using TubeTV to download videos from YouTube to play on my iPod during my commute.
TubeTV is a new freeware program for Mac OS X Tiger and above which enables you to search for, save, and convert Google Video and YouTube videos to a format suitable for playback on your favorite devices.
A built in search bar and web browser helps you conveniently locate your favorite videos, or, if you prefer, jump to the current URL of your default web browser with the jump button, or paste one in directly. High quality video conversions using the H.264 codec are performed with QuickTimeÂ®. Double clicking on a converted video file will open it in iTunes allowing easy library addition and syncing to your desired device. Note that TubeTV does not interact with iTunes directly – it’s the same as importing any other video file to iTunes. Alternately, just drag a saved video file on to QuickTime Player to view it locally.