The Open Toolkit is an advanced, cross-platform, C# OpenGL, OpenAL and OpenCL wrapper for Mono/.Net. It is especially suitable to RAD development and can be used in games, GUIs (WinForms, WPF, GTK#) and scientific applications.
The official repository can be found in github:
https://github.com/opentk/opentk
The sourceforge repository will no longer be updated, but will remain online for posterity.