This scripts works well under Greasemonkey in Firefox 4.0.1 - it adds a list of download links, one for each format.

However, it does not work in Opera 11.11. Could the script be modified to make it work under Opera's user script model? See http://userscripts.org/scripts/show/65477
"Yays! (Yet Another Youtube Script)" as an example of a script that detects the browser and runs the script using the appropriate code.

I tested this using the 15 second short video at http://www.youtube.com/watch?v=CXtk06d27Jg

The script added these four download links:

DOWNLOADMP4 HD(22)
DOWNLOADFLV HQ(35)
DOWNLOADFLV(34)
DOWNLOADFLV LQ(5)

Each link worked and gave a file playable with VLC.

EDIT: just to note that I have just tested is a userscript that works in both Firefox and Opera 11.11 at
http://userscripts.org/scripts/show/25105 .

i like it . this is best kind of Dll link for youtube

Well done on designing this userscript for youtube. I'll provide a detailed usage guide on features to my readers at http://www.crimechamber.com