I use it for gamedev (xna/monogame + xamarin basis == my game can run on pc, win8 tablets, windows phone, iphone/ipod, ipad, android)
i use it for tools (wpf)
i use it for wp8 and win8 apps.
i use it for server side code (mostly nancyfx based, on asp.net or owin).
i use it for local windows services (typically nancyfx, selfhost)
i use it for tiny tools (command line apps that do something quick for me)
i use it for realtime game server communication (through signalr)
i use it for raytracing (no simd and no gpu == comparably slow, but i just love doing it).
i could use it on linux and osx (mono), and even on tiny controllers like arduino-thingies (.net micro framework).
i could use it to write (most of) my own os, if i ever have too much time
so it's very flexible.