Modify every aspect of your favorite websites
If you have some basic programming skills, you can adapt a website just the way you want it by creating scripts. Greasemonkey makes the process a lot easier by enabling you to both find and create scripts online.
For example, you can change the design of YouTube to give it a very elegant black look. Or, if you want to download photo albums of your friends in Facebook, simply install a Greasemonkey script that adds a download link on each photo. Alternatively, want to turn the background of the Google search page to black? Simply install a Greasemonkey script and the color changes instantly.
There are many other useful scripts that allow you to remove ads from some sites and optimize page loading of popular sites such as Gmail , Flickr, Wordpress, MySpace, Digg etc. To install a script, just click on "Install this script" but if you have programming knowledge - why not create your own? You can do just about anything you want.
Greasemonkey is an excellent way of customising pages in Firefox.