3DMath contains utilities for working with 3D space using vectors, quaternions, matrices, and frustums.

For information on vectors: http://sacredsoftware.net/tutorials/Vectors/Vectors.xhtml
For information on quaternions: http://sacredsoftware.net/tutorials/Quaternions/Quaternions.xhtml
For information on matrices: http://sacredsoftware.net/tutorials/Matrices/Matrices.xhtml