Sharp3D.Math

[Hypnotron]

http://www.codeplex.com/Sharp3D

haven't checked it out yet but its a math library written in c#

[Zaknafein]

This leaves me wondering what's the public for this library. Anyone using XNA would be using the built-in Matrix/Plane/... types, which have a lot of methods and operator overloads et al. They're really well-done.
And if you're in TV3D, you've got TV_3DVECTOR and TVMathLibrary and such. Usually 3D engines provide their preferred implementation of vector math types.