This has been hitting a lot of waves lately, on Hacker News, Product Hunt, Reddit, etc. Still waiting for my beta invite, but I think it's worth the wait.
Some info from their website: DuoCode is an alternative compiler, powered by Microsoft Roslyn, and integrated in Visual Studio.
It magically cross-compiles your C# 6.0 code into high-quality readable JavaScript code, enabling rapid development of web applications utilizing the extensive features of the C# language, the Visual Studio IDE, and the .NET Framework base class libraries.
Development in C# with Visual Studio brings great productivity wins, thanks to strong-typing, code completion, compile-time error checking, static analysis, code navigation and refactoring.
Develop HTML5 applications using strongly-typed and documented class-definitions of the entire DOM class library (including HTML, CSS, SVG and WebGL definitions).