StumbleUpon is one of the coolest tools I've seen for a while on the web. Install it as a Firefox extension, and one click of the "Stumble!" button will take you cool websites in categories you select when you sign up. So far, I've stumbled upon a timeline of art history, a online planetarium, some cool animations, and a dictionary of British slang.
I warn you. Do not install this right now if you have a lot of work to do. Seriously, this can totally be a time suck.
NP: "Eighties" - Killing Joke