I recently installed a firefox addon called greasemonkey, and have been loving it. It allows you to install user-made javascript snippets that enhance websites in some way or another. Lifehacker has a neato list of useful scripts, so you don't have to trudge through userscripts.org to find good ones, and right now I'm using Profanity Filter, Greased Lightbox (makes google image searches way better), and youtube dimmer. What scripts, if any, are you using?