Microcosm, which is kind enough to host this site's message
The Microcosm is a new site, but shows much promise.
and it's associated message
The official Planescape site, Planewalker.com has lots of
stuff, though it's all from second edition. Still, there are tons of
ideas and edition-agnostic material there.
Which hosts this site. It's not super fast, but for
the price (free) you can't beat it.
Planescape forums at RealmsofEvil.net
A very active Planescape forum.
Was once the official Planescape site. It isn't updated anymore,
but still has tones of chant.
The City of Doors Initiative
An incredibly ambitious Neverwinter Nights module.
Maintained by Rip Van Wormer, it's a good place to find a ton of
interesting treatises on various Planescape topics.