Accord.NET Framework - Machine learning, computer vision and statistics framework for .NET
FastCV Computer Vision - FastCV will enable you to add new user experiences into your camera-based apps like:
BoofCV - BoofCV is an open source library written from scratch for real-time computer vision.
ImageUltimate - Fastest and easiest ASP.NET Image Resizer
OpenCV - OpenCV is the world's biggest computer vision library
SimpleCV - SimpleCV is an open source framework for building computer vision applications.