YouTube recently started offering video playback in higher-resolution, better-sounding formats like MP4, and that bump in quality can now be downloaded for desktop use as well. The Google Operating System points out two easy methods for grabbing files: A bookmarklet that adds a "Download as MP4" link to video pages when clicked, and a Greasemonkey script that automatically creates the link. Both require that you right-click and assign the to-be-downloaded file the ".mp4" extension, and both may violate YouTube's terms of use, but, as blog author Ionut points out, the same files are available in your browser cache after watching. Hit the link to grab the bookmarklet and Greasemonkey script for Firefox (and Opera) users.
Download YouTube Videos as MP4 Files [Google Operating System]