vvvv - visual live-progamming for .NET

vvvv is a node-based visual programming environment whose programs run on the CLR (same virtual machine as C#). It has hot reloading (compilation happens in the background) and can be extended with custom nodes written in C#. It has built-in libraries for 2D and 3D rendering, image recognition, machine learning, networking, and IO for various devices (Kinect and cameras). It's been used for a lot of interactive art installations, as seen in the "showcase" section on the website.