Netduino is an open source hardware platform that runs applications built with the .NET MicroFramework. Applications can be built on Windows (with Visual Studio), or on Mac OS (with Xamarin Studio). Netduino can be used to build nearly any Connected Thing you can dream up.
It’s similar in concept to the Arduino platform, but is generally more powerful and instead of writing applications in C/C++ or Wiring (essentially, C++ without header files), applications are written in C#, which brings powerful, high-level language constructs to the toolbox such as threading, event handling, automatic garbage collection, and more.