Guy's Polyhedra Pages http://www.queenhill.demon.co.uk/polyhedra/
Vertex figures, filling, faceting diagrams, stellation, defining polytopes through generators, trimethoric and trisynaptic polyhedra, space-filling polyhedra, lost stellations of the icosahedron, and links.
The Charged Particles Model http://presh.com/hovinga
Polytopes and optimal packing of p points in n dimensional spheres. Contains a java applet based on a model which allows for generation of multidimensional regular and semi-regular polytopes.