MSN Messenger server farm
One of the more interesting entries from
Paul's WinHEC 2005 blog: "
The MSN Messenger server farm handles over 70 million concurrent sessions every single day. Until recently, they were using 250 32-bit servers to manage that load. When they switched to Windows Server 2003 x64 Edition running on x64 hardware, they reduced the size of the farm to just 25 servers"