Many Fortune 500 companies "limit" themselves to windows, I will stick with the guys making money, rather than the guys who do it as a hobby.
the top 6 domains in terms of page-views in February according to Media Metrix were:
1) Yahoo, 2) MySpace, 3) MSN, 4) Ebay, 5) Google, and 6) Hotmail.
P.S. 3 of the top 6 sites (MySpace, MSN, and Hotmail) run on IIS and Windows.
P.P.S. the other 3 sites (Yahoo, Google and Ebay use microsoft technology for parts of thier sites!)