Greasemonkey is an extremely versatile add-on for Mozilla Firefox which allows users to install scripts that can make modifications to most HTML-based websites. Greasemonkey can greatly enhance a website by changing the layout, adding new features, or increasing ease-of-use.
To install Greasemonkey, head on over to the Firefox Add-On Greasemonkey page and click “Add to Firefox”. After restarting your browser, Greasemonkey will be active and ready to accept scripts (it should be noted that Greasemonkey will have no installed scripts by default).
Want to find some great Greasemonkey scripts? Check out userscripts.org (sorted by script popularity) or search for your favorite websites there. Some of my personal favorites are the Inline MP3 Player (automatically adds a flash-based player with controls to MP3s) and Textarea Backup (automatically saves textarea information in the event of a browser crash or accidental navigation away from the page).
Have a great Greasemonkey script? Share it in the comments!