Greasemonkey

Greasemonkey improves just about anything it touches, whether it's adding must-have features to Gmail, streamlining your social life in Facebook, or speeding up your blog posts The best part? Thousands of Greasemonkey scripts are freely available for download, and installing them is literally as simple as clicking a single link.

Note: This link takes you to the add-on's page at Mozilla.org, where you can automatically install the file into your Firefox browser.

--Adam Pash