Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make persistant changes to specific web pages. You can alter the layout of a page, disable the advertising, add links to other services.
The screenshot shows Gmail Saved Searches script in action.
Use Greasemonkey (required: install Firefox)
1. Download Greasemonkey
2. Find scripts
3. Learn more about Greasemonkey (free ebook with videos)
4. Greasemonkey hacks (get the code)
5. Greasemonkey hacks (buy the book)