Get it now from Mozdev. You will not be disappointed.
I’ll let Mozdev explain what it does:
This is a great webpage for user scripts, the extensions that Greasemonkey hooks into the browser itself. It’s a long list broken down by which website the user script is aimed at, although many of them are general-purpose.
Dive into Greasemonkey isn’t the best introductory work I’ve ever read, but it does help you get your feet wet in writing your own user scripts. It is freely downloadable in HTML and PDF.
(Speaking of extensions, you can compile Greasemonkey user scripts to standalone Firefox extensions. Which is nice because it’s a lot easier to write a user script than a full extension.)
Greasemonkey is generating buzz. Some people dislike the level of control it gives users, apparently, but I think it’s long overdue.
So try it out and have fun.