Update
Out with the old, in with the new. It has been quite a while since this article was written, and it was seriously out of date. I have updated the bookmarklets, included here for convenience. Be sure to use these links here at the top, and click and drag them to your bookmarks bar.
More information about this newest version can be found in my post detailing them.
I’ve spent a couple hours looking at YouTube code and videos and have updated my bookmarklets accordingly. Instead of only offering HD and SD download of MP4s this defaults to download the highest available quality of the specified type: FLV or MP4.
They’ve been tested only on Firefox 3.0.4 on Mac OS X 10.5.5, but I’ve no reason to believe that they won’t work in any browser on any OS.
Enjoy!