I think most of us who favor a particular setting are aware of other websites devoted to that setting, but that may not be true universally. And if we'd like to find out more about another setting, we'd probably be completely in the dark. So I thought it might be helpful to list some of the setting-specific sites. I'll list the few that I know, but I hope that others will add to the list.
EDIT: Here's what we've all come up with so far. I've also added links to the Wikipedia articles about each setting.
Multiple worldsThe PIAZZAhttp://worldsofdnd.com/forum/index.php (especially Eberron, but also includes other worlds)
Worlds of TSR forum at Dragonsfoot.org (single forum for all settings, including post-Gygax Greyhawk)
BirthrightContent: Birthright.netForums: Birthright.net forumsBlackmoorblackmoor.mystara.us Dark SunContent: The Burnt World of Athas (Official fansite)
Forums: The ArenaDragonlanceContent: The Dragonlance NexusForums: The Dragonlance ForumsThe KencyclopediaAdlatum Campaign Setting (unofficial fan expansion for Dragonlance)
Dragonlance.com (home of the d20 Dragonlance product line)
EberronEberron Campaign Setting Homepage (Official WotC webpage)
Bossy the Cow (site belonging to Eberron creator Keith Baker, aka Hellcow)
http://www.online-eberron.com/Eberron forums at worldsofdnd.comForgotten RealmsForgotten Realms Campaign Setting Homepage (Official WotC webpage)
Content: CandlekeepForums: Candlekeep ForumsNorthern Journey (free FR adventure campaign)
http://www.forgottenrealmsvault.com/GreyhawkContent: Canonfire!Forums: Canonfire! ForumsMaldin's GreyhawkAnna's Greyhawk mapsGrodog's Greyhawk linksHarvester's HeroesGreyhawk Online (free webhosting for Greyhawk related websites)
Greyhawk webringGary Gygax's World of Greyhawk forum at Dragonsfoot.org (dedicated to "Gygax's vision of the setting")
Living Greyhawk sites
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MystaraVaults of Pandius (Official Fansite)
Mystara forums at The PiazzaThe Mystara Mailinglisthttp://mystara.thorf.co.uk/ (Mystara Maps)
http://www.mystara.fr/ (Maps, files)
PlanescapeContent: Planewalker.comForums: Planewalker.com forumsThe MimirPlanescape webringPlanescape sketches on Tony DiTerlizzi's blog (not strictly a Planescape site, but of interest to fans)
RavenloftContent: Fraternity of ShadowsForums: Cafe de NuitSpelljammerBeyond the Moons (Official fansite)
Additional Spelljammer sites
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One more thing: Since The Piazza is listed at the top, under "Multiple Worlds," I was going to remove the links to the world-specific forums under Mystara and Spelljammer. Howerver, I see that the main/official sites for those worlds do not have forums of their own, like the Dragonlance Nexus, Canonfire!, Planewalker, Fraternity of Shadows, Athas.org, and Birthright.net do, so I decided to leave the Piazza links under those two worlds. However, I'm not planning to add similar links for the other worlds, as they are all easily reachable through the main Piazza link at the top. That seems like the best way to handle the situation.