Computer Vision Library

I found EmguCV and AForge handy.

EmguCV is the .NET ported version of OpenCV, actively maintained (using the latest OpenCV at the time i checked)

AForge used to be my personal favorite, it's developed ground up by Andrew Kirillov (sadly he announced the end of free public support on April 2012). Well documented and very intuitive.

A friend of mine has used OpenCV.net (a C# wrapper for OpenCV) for his Master's thesis (which was about hand gesture recognition) and found it very well developed and easy to use.

I will add my little answer. For computer vision project in .net I usually use OpenCVSharp. It's OpenCV wrapper which complex wrap opencv function and it's under BSD 3-Clause License. It has a wide use and is usually sufficient in projects.

Another usefull tool is ImageMagic which has .NET interface. In computer vision I use this tool to make operation on single frame.