C# Vector Library

XnaGeometry
Here's a free, open source, 3d vector library for C#. It's ported out of the MonoGame project, MonoGame's licensing is liberal so it's legal. Mono is a drop in replacement for Xna so you can use the Xna docs. MonoGame is huge and constantly in development, you might not need all of the features. I've changed all the floats to doubles, you can change them back with a find/replace if you want. If you want to decouple your calculations from the renderer, or you need double accuracy, or you just need the math and not the renderer, then this might be the way to go.
Classes: Vector2, Vector3, Vector4, Matrix, Ray, Plane, etc...
XnaGeometry