httpwebwitch - 10:00 pm on Oct 9, 2009 (gmt 0) I'm only guessing.
Any music & video streaming portals like Grooveshark, Break, Onion, et al would be heavy bandwidth users, but I doubt any of them come close to youtube. Facebook and MySpace are likely in the top echelon of bandwidth consumers. Yahoo, too.
I'm only guessing.