WikiPedia. And TVTropes, though I don't really go on there that often (but when I do, I'll spend a lot of time there). Both of those websites just...ensnare you. You go there, then whoa! A colourful word! You click it, and hours later you find yourself on an article that isn't even remotely connected with the article you first read.
Well, I do believe everything is connected (in this case the subjects of the articles), but still. It's amazing how far you can go.
Also, webcomics. I can read them for hours. I usually, when I start readin one, read for as long as I go to sleep (unless it's a new one I start reading that is absolute shit).