All sorts of fun things.
Lets you block ads.
Blocks ads, gets rules from some central lists.
Lets you switch between multiple proxy configurations. Useful if you sometimes tunnel through an ssh socks proxy.
User Agent Switcher
Lets you lie to web servers and say that you're using a browser other than Firefox.
Lets you tweak how a page looks one time only. Useful for preparing a page for printing.
Lets you see the html tags around the parts of a page. Useful for seeing how it's written.
Lets you write scripts that will change how particular web pages are displayed.
Lets you create Greasemonkey scripts in a Wysiwyg fashion.
Lets you tamper with requests before they are sent out.
Lets you record and playback macros for web interactions.