FFG:Portals and Planes (Link to PDF version, also in print): A fine planar product by Mike Mearls with rules for tweaking races that live on other planes, prestige classes, ideas and rules for portals, planar pathways, and planes, and some great creature ideas. The PDF is a steal at $5.
Mongoose:
Classic Play: Book of the Planes
(Link to PDF version, not as cheap as the above, look for the print version...) A nice book with planar organizations, rules for planar magic, shaping and creation, a more detailed plane description system.
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