This is kind of an interesting tip, courtesy of the Google Operating System blog:
An interesting side-effect of YouTube's recent push for higher quality videos is that most videos can be downloaded as MP4 files directly from YouTube. Until now, you could only get FLV files from your browser's cache or using one of the many websites that let you download YouTube videos. In fact, to download the MP4 files, you need to use the same URL like for FLV files and append "&fmt=18":
http://www.youtube.com/get_video?video_id=ID&t=SIGNATURE&fmt=18
ID is the video's identification value, SIGNATURE is a value that prevents you from downloading the file just by knowing the ID. You could create this URL by looking at the source code of a YouTube page, but it's much easier to automatically generate it.
One way is to save the following link as a bookmarklet by dragging it to your Links bar (in Firefox, Opera) or right-clicking and adding it to your favorites (in Internet Explorer)....
I wish all the online video sites would switch to MP4. There's nothing more useless than a FLV file. [Ponders that comment for a moment...]