All these links should work now. Tell me if they don't, or if you find sites I didn't mention. Or, those of you who are moderators should feel free to edit this yourselves.
General Reference
A Tiefling's Exultation:
http://www.geocities.com/Area51/Vault/9974/index.htm
Li Po:
http://www.pathguy.com/planes.htm
On the Wings of Mephits:
http://www.deathstar.org/~krlipka/ps/index.html
Travels and Travails:
http://www.ruined.net/halls/travails.html
The Shadowlie:
http://www.geocities.com/domrot78/shaplane.html
Ootheca:
http://www.geocities.com/magnumtb/page3.html?1075063343890
Dreams of Dreams:
http://www.nondeterministic.org/dreams/
Planewalker:
http://planewalker.com/index.php
Old Mimir site:
http://geocities.com/athens/7117/
New Mimir site:
http://mimir.net/main.shtml
Fiction archive:
http://www.deathstar.org/~krlipka/ps/fiction/local.html
The Third Mimir:
http://exhalus.net/bguard/mimir.asp
...unless Infinity:
http://www.geocities.com/TimesSquare/Arcade/2894/ps/
Scripta Planorum:
http://www.geocities.com/SouthBeach/1482/ps/ps.html
Ripta Planorum:
http://www.geocities.com/ripvanwormer/riptaplanorum.html
Planar Portals:
http://www.geocities.com/Athens/7117/zportals/
Ekkaia.org Planescape page
http://www.ekkaia.org/rpg/dnd/ps/
Map of Sigil:
http://www.shadowland.org/Planescape/maps.html
Jason Ng's Kriegstanz
http://members.shaw.ca/jykng/
NPC database:
http://www.aarg.net/npc/aargplanescape/list
Planescape Central:
http://www.geocities.com/Yogiland_Central/PlanescapeCentral.html
The Dark Eight:
http://emeric.sarron.free.fr/DarkEight.htm
Gradasso:
http://www.geocities.com/gradasso/planes/TheGreatRing.html
Sigil: City of Doors:
http://www.geocities.com/thes_33/sigil.html
The Ordial Plane:
http://pages.prodigy.net/shamilto/ordial/oplane.htm
Blue Troll's netbooks:
http://www.fortunecity.com/victorian/byzantium/55/worlds.htm
Outlandish oddities:
http://www.geocities.com/outlandishoddities/
Song of Endless Planes:
http://xargreyhawk.tripod.com/SongCrown/worlds.html
Portalseeker:
http://www.portalseeker.com/
Walking the Planes
http://www.geocities.com/Area51/Dunes/4656/Planescape/Planescape.html
mgrasso diversion
http://www.homegame.org/mgrasso/diversion/planescape.html
Sigil Prep School
http://www.sigilprep.com/index.htm
The beheaded fiend inn
http://www.geocities.com/Area51/Hollow/5279/fiendinn/index.html
Planescape NPCs
http://www.validus.demon.co.uk/nw/downloads/planescape%20npcs.doc
Of Darkest Gray
http://darktragedy.de/gray/
The Clueless Inn
http://www.geocities.com/findorian/index.html
A'azax (Abyssal layer)
http://aazax.com/
The Basics of the Language of Chaos
http://otherworld.aphid.net/xaos.html
Deities
Servants of the Seldarine
http://www.selvarin.com/servants_of_the_seldarine.htm
Gods of the various worlds
http://www.geocities.com/filbypott/gods1.txt
Gods from Earth's myths and legends
http://www.geocities.com/filbypott/gods2.txt
Nonhuman Gods
http://www.geocities.com/filbypott/gods3.txt
Campaigns
Blood War: Cursed Earth
http://www.bloodwarcursedearth.com/
Apotheosis (Orri's campaign):
http://www.hi.is/~orrie/apotheosis/index.html
"Squaring the Circle" revised
http://web.archive.org/web/20010429060935/http://www.geocities.com/SoHo/Exhibit/7210/squarecircle.html
Post-Sigil Planescape
http://www.harlekin-maus.com/postsigil/postsigil.html
Hellbound revised
http://pages.sbcglobal.net/zimriel/RPG/Hellbound/
A campaign story "arch"
http://www.greylight.com/glpc/planescape.html
Shemeska's Storyhour:
http://www.enworld.org/forums/showthread.php?t=77613
Evergold Portal
http://evergold.iwarp.com/
RPGarchive adventures
http://www.rpgarchive.com/index.php?page=adv&sysid=7&title=D%26D%20Planescape
All Doors Close:
http://webpages.charter.net/ericnoah/noahrpg/alldoorsclose.htm
Mechanics
Zach Shuford's Kriegstanz:
http://www.kriegstanz.com/
"Fractious Factions" web enhancements:
http://planehopping.tripod.com/287.html
Planescape Lore:
http://www.nightwasp.com/daggerdale/pslore.htm
Samloyal's Sensate Feats Page
http://www.geocities.com/cognitive_dissidents13/Sensates/Sensates01.html
Roaming Genasi Inn
http://users.erols.com/aburner/planescape/
Torment monsters:
http://www.gamejag.com/planescape/cast/prods3.htm
Planetouched:
http://members.shaw.ca/jykng/planetouched.html
Ken Lipka's player guide:
http://www.deathstar.org/%7Ekrlipka/ps/sense/playguide.html
Omar's Planescape: (GURPS and FUDGE)
http://www.sonic.net/~rknop/Omar/planescape/
Donjon Planescape:
http://www.harlekin-maus.com/donjon/donjon.html
Non-tabletop gaming
Faces of NWSigil
http://nwsigil.tripod.com/
Planescape: Vengence
http://www.planescape-vengeance.net/
Planescape: Embers of the Past
http://castellar.roguepenguin.com/Embers.htm
Planescape conversion for Ultima Online.
http://uo.thinkbht.com.
City of Doors Initiative
http://www.city-of-doors.com/
Forums
Planewalker forums:
[url]/forum]
Umbrasa Gravelands Planescape forum:
http://greyhawkonline.com/pitsofevil/v2/YaBB.pl?board=planescape
WotC Planescape board:
http://boards1.wizards.com/forumdisplay.php?forumid=292
WotC Deities & the Planes board:
http://boards1.wizards.com/forumdisplay.php?s=&forumid=98
WotC Forgotten Realms Deities & Planes board:
http://boards1.wizards.com/forumdisplay.php?s=&daysprune=-1&f=454
Planescape mailing list archives:
http://oracle.wizards.com/scripts/wa.exe?S1=planescape-l
DiceFreaks:
http://dicefreaks.com/phpBB2/index.php
Kismet's Gamer Gathering Planescape Board
http://s8.invisionfree.com/gamergathering/index.php?showforum=19
Art
Tony DiTerlizzi
http://www.diterlizzi.com/
Planescape: Torment Artwork
http://www.rpgfan.com/pics/planescape-torment/art.html
Planescape: Torment Concept Art
http://www.planescape-torment.de/konzept2.htm
Ultimate Fantasy and SF Gallery
http://draax.free.fr/
Maps of Sigil
http://www.dungeony.cz/planescape/planescape.php
Sporeboy! Planescape Fan Art
http://www.sporeboy.com/Sporeworx/sporeworx.html
Carceri Exiles
http://www.operatique.com/planescape/index2.htm
Heatherwind Planescape Fan Art
http://www.heatherwind.com/gSPC_Fanart.html
The Sensoriums
http://mitglied.lycos.de/rpgart/
Vicki Hood
http://www.aminita.co.uk/illustration_fantasy.htm
Ken Lipka's Planescape Fiction Archive/Artist's Corner
Noah Stacey's elfwood gallery and his deviant art gallery. I like his crazy cityscapes. He's also got stuff at Ken Lipka's site.
Stephanie Pui-Mun Law's Mythology Gallery. Maybe a bit too precious, but creative and undoubtedly mythological.
Visionary Art in France. Visionary art from France. Also other breathtaking surreal art hosted on the site.
Tom Baxa. He's painted or inked a number of Planescape monsters here and there. He illustrated the old Monstrous Compendium Outer Planes Appendix, but I think he's improved as an artist dramatically since then.
Randy K. Post. He did some Planescape covers and illustrated much of Dead Gods.
Adam Rex. He did much of the later interior art after DiTerlizzi left. He painted the color parts of Dead Gods, and my favorite monster illustrations in the Planescape Monstrous Compendium Volume III and the 3e Monster Manual.
Robh Ruppel. Lots of cover art, and he co-illustrated the comic book inside Hellbound.
Gerald Brom. He did the cover of The Inner Planes. His art is great inspiration for Doomguard and Dustmen.
Anthony Waters No real Planescape connection, but Dana Knutson is his art director and he paints elementals and angels of death.
Brian Despain. He did some art in the Planescape Monstrous Compendium III. That was pretty shoddy, but the art on his site is really good and interesting in a quirky way, the way I like it.
Douglas Carrel. Fantastically weird creatures.
Scott Fischer. Monsters, humanoids, and Asmodeus.
Dave Allsop. Darker and weirder than anything I've linked to so far. Or, at least more weirdly dark.
Claudio Pozas. His Planewalker character design.
Can't forget Brian Froud. One of DiTerlizzi's forefathers.
Giovanni Piranesi Check out the images labeled "Carceri" - they're what David Cook was looking at when he was dreaming up Sigil.
Various links I haven't looked through
Planewalker.com's Artist Alley
Caries-Caeli. He's recently started posting in the Planewalker forums.
Chris Appelhans. You can see his early work in the Mimir's art gallery. Now he's mature and professional and does a lot of dark surreal urban fantasy.
Planewalker.com's Artist Alley (can't believe I forgot this one)
Caries-Caeli. He's recently started posting in the Planewalker forums.
Chris Appelhans. You can see his early work in the Mimir's art gallery. Now he's mature and professional and does a lot of dark surreal urban fantasy.
Rob Thomas. Demons and undead and such.
more Piranesi. More surreal architectural cages and prisons.
David Mitchell. Sigil in graphite.
Erol Otus (Otus was a 1st edition illustrator; he's got a wild, cartoonish style that I think is pretty interesting)
The Dark Wall
http://www.geocities.com/thedarkwall/
Neon Rainbows
http://www.geocities.com/neon_rainbows/dd_pics.html
Webcomics
Unity of Rings (the official Planescape comic)
http://www.wizards.com/dnd/DnD_PS_Comic.asp?Page=1
Pihakwa. The Shepherds of Googie are the best fiends ever.
Kangarou. Manga-style adventures in parallel worlds and nightmare regions of madness and the mind. With demons that look like evil toys.
Fallen. Fallen angel, Chinese gods.
Copper. Fantastic cartoon dreamscapes, mostly upper planar but with some darker regions.
Pipingrad. Look at the sketchbook. I can imagine ghost dog's creepy little blobs skittering around Sigil.
Serenity Rose. One of the character designers for Invader Zim. Look at the snarlies gallery.
Planescape-themed webcomic
http://swiftbow.keenspace.com/
Demonology 101. A webcomic about half-fiends, but I'll call them tieflings for convenience. They live in the 21st century, but it's still a very interesting look at the politics of the lower planes. The "magdelines" have one of the same powers kytons have.
Slightly Damned
http://www.raizap.com/sdamned/index.htm
Potato with a Mohawk (Primus' webcomic)
http://potatowithamohawk.blogspot.com/