Here you find which Sportsbook is receiving the most traffic, this is according to Alexa which  ranks sites traffic on a three month average and is updated on a monthly basis.
Ranking done on 11th November 2007.

Quoted Sportsbooks

** Note some website use one main site for Sportsbooks, Casinos and Poker Rooms, which may slightly distort the figures. Bwin, Bet-at-home, Centrebet, Bettson, Npbet  are not ranked as their home page is https