Greasefire
Greasefire is a browser extension for Firefox and Chrome that finds relevant user scripts from Userscripts.org for the pages you visit.
Downloads
The Firefox extension can be downloaded at https://addons.mozilla.org/en-US/firefox/addon/greasefire/
The Chrome extension is not yet complete, but grab the source and give it a try!
Discussion
Discussion forum on userscripts.org: http://userscripts.org/forums/5