Links
Planewalker.com. The official internet home of Planescape. Go there.
Mimir.net. The former official internet home of Planescape. Rarely updated these days (because Jon's working on a PhD), but the archived material is excellent.
Wizards of the Coast. The former publishers of the Planescape campaign setting. (Oh, yeah. They also publish D&D, and several other campaign settings. But they aren't as important..)
This site consists of scans of the artwork of Tony DiTerlizzi, one of the conceptual artists for the Planescape setting.
Portals: Paths of the Planewalkers. Beautiful site. Simply beautiful. And it contains an enormous database of Planescape links.