userscript.org

Greasemonkey is a plugin (referred to as an ‘extension’) for the Firefox
browser. It allows you to change how your favorite pages behave and
look. There are many scripts that have already been written, and if you
know javascript you can easily create your own! This site is a
repository to download and install Greasemonkey scripts.
And Userscripts.org is a repository for … userscripts! This is still in development, so … have fun!
I also created one userscript here
Another interesting extension is platypus
Platypus is a Firefox extension which lets you modify a Web
page from your browser — “What You See Is What You Get” — and then save those
changes as a Greasemonkey script so
that they’ll be repeated the next time you visit the page. Editing pages to suit
your needs is dandy — but making those changes “permanent” is the real payoff.