.NET framework is Microsoft‘s take on Java - a single VM for multiple plaftorms (essentially multiple versions of Windows, until Mono came along) that can be targeted by several different language environments.
Damn impressive piece of engineering, and slightly better (architecturally) than Java.
- Uno - a cross-platform app stack atop Xamarin.Native
- vvvv - a visual live-programming environment
- SciSharp - a set of data science libraries inspired by the Python ecosystem
- VG.NET - vector graphics library in managed code
- Pocket C#
- Creating Mac OS X applications with MS Visual Studio Express - grab VSE, install Mono, and build an app bundle around a wxWidgets executable.