well, for starwars there's FFURG (Forgotten Force Ultimate Resource), in the forum there are sections for star wars customs and a section for miscelaneous figures. They also have customizing guides. I think rebelscum.com also has some custom material.

fighting118th.com has (general) military customs and dioramas. Tri-gate had a really cool forum, with customs of al topics (fantasy, sci-fi, military, etc.), to bad it's closed.