By far the best out there when it comes to download scripts! Integrates perfectly with the YouTube site style, the button is exactly where you'd expect an official button, I can't imagine it more easy and better designed.
And it always updates quickly!
I don't what they recently changed at Youtube, but it ended up breaking almost every Youtube script I had.
This was first choice after http://userscripts.org/scripts/show/25105 ceased to function. I am using it with FF5 Ubuntu edition along side Loop Youtube 1.1.
It works great and I seriously am glad for the best quality available setting. From experience with the script /25105, the highest quality MP4 is usually the first URL, though that wasn't the URL it would grab. :\
Thanks a bunch. Great stuff.
This is not a free script, as far as I can tell. When I clicked the download button on youtube, after downloading the script, it launched Quicktime, and wouldn't allow me to save the video until I purchased Quicktime Pro.