Hyperspace Travel Guide
My personal page explaining the math behind the projection of four dimensional objects to 3D. Also, a tiny zoo of four dimensional objects. It is quite badly maintained, I'm afraid.

Uniform Polytopes in Four Dimensions
George Olshevsky's catalog of polytopes, the four dimensional analogues to polyhedra in three-space. Contains many interesting objects and some 3D polyhedra too.

The Fourth Dimension
A page by John Savard which tries to make some polytopes imaginable, most notably the 120-cell. IMHO, it succeeds quite well at that attempt.

4D Geometric Objects
A catalog of four dimensional object with an emphasis on rotopes. The same site also hosts a little 4D FAQ and a HOWTO on Visualization fo four-dimensional geometry.

Fourth Dimension: Tetraspace
A forum, created by Garrett Jones, dedicated to the discussion of four dimensional space. The posts usually have high quality, you can learn a lot here. The page also hosts an Introduction to "The Fourth Dimension" and the Higher Dimensions Database.

Uniform Polychora
This is a site by Jonathan Bowers, cataloguing polychora. I haven't had time to check it out yet, but it enjoys a good reputation.