I stumbled today at a page thinking it was a joke until I checked it manually to make sure it was correct. The page is simple. It is from NetCraft and shows the servers that are being used by big companies like IBM and Microsoft.
Logically speaking, Microsoft should be using Windows Servers at least to prove to its clients that Microsoft servers are stable everywhere and for everything. I was shocked to find out that Microsoft was using Linux (its competitor) as a server.
The screenshot below is from NetCraft and can be seen (while it exists) by clicking on this link.