A friend is starting a project that will have a fairly major chunk of math involved (AES encryption, although I know nothing more than that), and would like to avoid having to reinvent yet another set of maths classes. Now the only C++ one I know about is the stuff in D3DX, most people using OpenGL seem to roll their own. What math librarys can people recommend? Thanks.

How about the MTL (matrix template library) ?

A friend of mine just implemented aes as part of his diploma-thesis, can''t say he needed that much matrix stuff to require a specialized matrix lib.