We run a windows os with iplanet to the outside world but that proxies over to our big important boxes which are *NIX (though we do have a couple of MSSQL servers for light weight apps. If not for politics I would replace our main webserver with Linux running apache tomorrow it would save me the trouble that 6 or seven reboots a year cause..
There's another one above Windows: Imagine if all the IOS boxes disappeared.