I just discovered a Greasemonkey Userscript that downloads YouTube videos as MP4. This cross-browser script adds an option to download YouTube videos as MP4 and FLV files. The script is working perfectly on my Firefox 7 and Chrome Browser.
Unlike many other scripts, this script downloads the videos directly from YouTube and doesn't connect to any other site. The script works in Firefox, Google Chrome, Opera, Safari and Internet Explorer browsers.
The script works in: