Squeaky Monkey

GreaseMonkey is the greatest. I knew this back when I installed it the first time, but the more I use it (or don't use it [noticeably]) the more I love it. I recently installed the Google Big Brother script and get a kick out of it.

GreaseMonkey is to the Internet as a pencil is to a piece of paper.

Or as their site says: "Because it's your web."