GreaseMonkey is a very powerful extension for Mozilla Firefox. It allows for users to create scripts "which lets you to add bits of DHTML ("user scripts") to any webpage to change its behavior." I use GreaseMonkey at work. Our portal and other internal pages don't render correctly on Firefox, mostly due to poor html formatting and applet embeds. I've talked to those responsible and showed them how to fix it but the "powers that be" won't allow it. After a bit of my own "monkeying" around I can view the pages just fine. I've also written an extension to a great script that removes ads from slashdot.org. The new and updated script fixes formatting issues from removing the ads and can be found here.