After hearing a lot about grease monkey I finally got around to installing it and playing with it, and it's pretty cool!.

What is greasemonkey?

It's simply a firefox extension that allows you to run JavaScript User Scripts when the current page matches a URL pattern specified. So you can basically customize any web page you want with it.

How do you install User Scripts?

Simply right click on them and click Install User Script, or visit the user script page and select it from the Tools menu. The script must end in user.js.

Here's an example, I made a user script that modifies weblogs.macromedia.com to show the URL of the entry when you hover over a link. You can get it here.

Another example - lets say you live in the UK, but everyone links to Amazon.com, my Amazon Localizer greasemonkey user script will convert any foreign amazon.com, amazon.de, etc links to your locale, amazon.co.uk in this case. I made a script for pretty much every Amazon locale, including US.