http://www.blackperl.com/javascript/greasemonkey/ft/
So after about the 100th time of trying to tag a site in del.icio.us that I’d already tagged before (early onset alzheimer’s?!? More likely the effects of my misspent youth), I thought to myself “Someone has got to have already built something that queries del.icio.us in the background and lets you know if you’ve already tagged a page.” And sure enough, someone had, using Greasemonkey.
This script (great name!) displays a small piece of text on the screen with the tags you used (and optionally, how many other folks have tagged it to) on any page you visit that is already in your del.icio.us links and then gently fades away (the fade time can be configured). I use the del.icio.us extension for Firefox already, and this would seem like a natural addition they could build into it, but until then, this script does exactly what I want. – SWL