edited May 2011
I am looking for a C# 3d math library for use in Photon, so I can have classes like Vector3 and Quaternion.
Which 3d math library is everyone using that is compatible with Unity?
I'm not sure about the licensing but you could use the assemblies that Unity provides:
In Unity Editor, you can "sync mono project". The solution will include a reference to Unity's assembly dlls. You should be able to use these in your server project, too.