Allows you to take absolute control over any website (on your end, that is - this won't let you steal!). Tired of those ad banners? Get rid of them! Background colors a bit jarring? Change them! And that's just the beginning; some of the things more advanced users will do with this extension are just astounding.

I love Greasemonkey. Here's what I use it for - a web site that I work with has a real problem with spammers "scraping" (ie stealing) it's content, displaying it on blogger blogs, and slapping ads up to make money. Invariably, these scrapers will hide the "flag this blog" tab on the Blogger template, making it very hard to bring the spam site to Blogger's attention. I little Greasemonkey script exists that unhides this flag button, making it easy to flag the splog for deletion.