Planewalker is a fan-created web site for TSR's old Planescape campaign setting. Based in Sigil, City of Portals, the campaign setting was released in 1994 and took players on a grand tour of the Inner, Outer and Elemental Planes, introducing them to realities that are changeable based on philosophy and belief. The web site hosts community forums, and extensive database of Planescape materials, downloadable reference guides, and much more.
WotC has also elevated a few other fan sites to 'official' status, although that might have been changed to 'recognized' status by now. Regardless, these other setting sites are definitely worth checking out:
http://athas.org/ - Dark Sun
http://www.fraternityofshadows.com/ - Ravenloft (Not "official")
http://www.spelljammer.org/ - Spelljammer
http://pandius.com/ - Mystara
http://www.al-qadim.com/ - al-Qadim
http://www.birthright.net/ - Birthright
Thanks for those sites -- I've visited some of them when I was working on various Knights of the Dinner Table "Summon WebScryer" columns, but I don't think I've ever been to the Mystara or Spelljammer sites, and I know most of these don't exist in Nuketown's Linkport yet.
I'll be interested to see how many of these sites try to adapt their settings to 4th edition. I imagine some will fit in just fine (Spelljammer, Dark Sun) but I have a hard time imagining a 4E Ravenloft.
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