Greasemonkey is something you will love while browsing the worldwide web. It can practically change the looks and feel of the sites you browse to what you like to see. I mean customization of sites. What a great thing, userscripts.org has tons of scripts that can help you browse the web the way you like it. You can practically change the looks of your favorite Gmail Inbox and have it display the attachment that a message carries without opening it. You can write your own scripts, too. Mark Pilgrim's definitive Greasemonkey guide, diveintogreasemonkey.org will show you how.
One of my favorites is the GTalk add-on in Firefox. You can run the neat messenger in the sidebar of Firefox, without having to install it. Give it a try, download the gTalk Sidebar