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That's news to me. Not that I disbelieve. Really interested. Can you point me to some use cases? Why do they not just use Windows?

Mostly on mobile I would guess. Xamarin has a showcase on their site: http://xamarin.com/apps .

Also every game made in Unity is using Mono, which is _a lot_ of games. The Playstation Vita and Wii U uses Mono as well for third party development.