[stdgy]

Honest question, not snark:

Who are some big C# developers that create major applications consumers (Not enterprises) use everyday?

You mentioned the paucity of major OSX/iOS developers, but I'm not sure the case isn't the same for C#/.NET.

[Margh]

My first thought is any game written in Unity. Works on desktops and android more or less out of the box and iOS with a little bit of tinkering.

[Joe4evr]

Supergiant Games (Bastion and Transistor) and Squad (Kerbal Space Program), to name a couple. And all the StackExchange sites are made on ASP.NET, if you want to count that too.

[stdgy]

The Unity-oriented stuff are good examples. The Stack Exchange sites came to my mind as well, though I was hesitant to count web apps.