OpenTK (The Open Toolkit) is a game development library for .Net/Mono with an elegant, yet powerful, API. It enables you to bring your ideas to fruition, leveraging the full power of .Net, OpenGL and OpenAL.
OpenTK needs no setup. It runs on Linux and Windows and works with all .Net languages, like C#, VB.Net and IronPython. Best of all, it is – and will remain! – free for both commercial and non-commercial projects.
OpenTK homepage: http://www.opentk.com