MP4 Downloader is a simple, cross-platform, open-source browser add-on that downloads videos from YouTube, Dailymotion, Facebook Video, and Vimeo as MP4 files. This add-on can currently run in Mozilla Firefox (as an add-on) or in any browser as a bookmarklet.
A good reason to download videos in the MP4 format is that this format is iPod-compatible without additional conversion. All videos downloaded using MP4 Downloader can be easily transferred to an iPod (or almost any other portable device). See the Sync to iPod page for details.