Planet ADnD		Planet ADnD is a good site for those looking for info on dungeons and dragons. Many netbooks and more.
Planewalker.com		Lots of information on the whole planescape campaign setting.
Visit the Mimir		Here's another Planescape site. Mimir.net, however, is focused on the city of Sigil in the setting. This site has anything you ever wanted to know about the city that's shaped like a donut.
Snail Outfitters		Snail Outfitters, another planescape site, has tons of gadgets and gizmos to make your planescape campaign more enjoyable. Really neat stuff here.
Planescape guide		Here's a good Planescape site for those who have no idea what's going on in their Planescape campaign. Gives all the background stuff for the Planescape setting.