The best sites:
Irony Games-The place to find a PBEM to play in!
Olik's AD&D pages-Lots of good AD&D stuff of all types, including theGreat Net Book Archive!
Bean's page'o'AD&D adventures-lots of good adventures
The World of Veslach-world, links, PBEM, very good page
ADnD.com-the AD&D archive. LOTS of races, classes, items, equipment, etc.
Bohavia-an extremely well-done AD&D PBEM where you can sign up to lurk
The AD&D Site Exchange-Well-sorted, quickly growing list of AD&D links 
Original AD&D Campaign worlds/places:
The World of Veslach-world, links, PBEM, very good page
Palanis-an interesting world with Word files of races, deities, and other stuff
The Legolas RPG AD&D page-lots of links too | Highmoon-a town | Green Spider's Web
Dragar's Homepage-campaign world Irial, magic stuff, etc. | The World of Eltire-campaign world
AD&D - The World of Tryll-a world with history, gods, house rules, etc. & jokes too
Realms of the Solstice Moons | The World of Maron-original AD&D world with game, races, etc.
 The World of Alloryia | The World of Irial | Dreams Afoot: Aralon Online
(world of) Tevlar | (world of) JADE | The Numberless Islands-Campaign world and AD&D game
The Golden Sea-world with pantheon, adventures, geography, etc. | Hexalon (AD&D world)
1001 Thurian Knights- house rules, PBEM game, and an AD&D world. | Morbia (AD&D world) 
Archives: Acid Dragon's Library of Role-Playing Materials
Olik's AD&D pages-Great Net Book Archive and more! | Dungeon Masters' Guild
The AD&D Archive-kits, classes, monsters, races, spells, etc.
Paroo's AD&D page-LOTS of netbooks, classes, monsters, spells, etc.
The Thieves' lair-New Adventure Ideas, Rules Modifications, Info on what AD&D is, etc.
Arthur Knight's Homepage, consisting of files, classes, a good links page, and much more.
Mandrikar's realm-good site with adventures, netbooks, artwork
Unearthed Arcania-home of the new netbook w/new spells, classes, monsters
Peta's AD&D page | Sarmanda's Castle-rules, races, classes, etc.
 Lord Callubonn's Fantasy Domain-Netbooks, Programs, Utilities, etc.
Vlad's AD&D page-netbooks, new classes, good links | Eric's refuge-netbooks, etc.
Mountain Orb RolePlaying-monsters, riddles, music, and more
TuDragons-freeware AD&D stuff | Dragonloft-spells, netbooks, characters, etc.
The Realm of Muad'Dib the Nightbringier-AD&D core-rules CD materials
Azumandias' Domain-Misc. AD&D stuff for players, DMs | AD&D-monsters&stuff
Jol Baton Peak-Downloadable modules, charts, tables, and DM utilities
Warlocke's Page-spells, etc. | Furyon's Realm-interesting info&images
The World of Titan-AD&D campaign world & PBEM | Dragon's Breath Lounge
The Brighton Dungeon-adventures, magic items, etc.
Qubrak Shata's AD&D page has lots of interesting stuff.
AD&D programs, links, netbooks, etc. | The AD&D files-Programs, character sheets, etc.
Manslayer's Dungeons & Dragons Archive-links, jokes, etc. | The Deepest Dungeon-programs, classes, etc.
Azumandias' Domain-Misc. AD&D stuff
NetRPG-LOTS of programs, netbooks, etc. (not just AD&D stuff)
Saxton's fantasy RPG page-shareware, NPCs, character sheets, etc.
The Vigilant Gryphon (RPG)-some OK programs/articles
Great Net Prayerbook-more priest spells than you could ever want
Great Net Spellbook-more wizard spells than you could ever want
Paladin's Stronghold-all about holy warriors, and more
The Realm of Gilheru-stories, worlds, heroes, armies, and more
The World of Phantas-Sea-stories, spells, etc.
DM resources: Dungeonmaster9's AD&D realm
  The Glass Tower
Bean's page'o'AD&D adventures-lots of good adventures
The Cave-CGI scripts that make characters, names, and other RPG-related stuff
Great River Games | Rich Staats' RPG articles | Mark's AD&D page-links, adventures, kits, etc.
DM's Retreat-non-internet AD&D game and other useful stuff
M. J. Young' AD&D materials-alignment quiz, original DM info, etc.
Custom RPG Art-for Gamers; Character Art, Monsters, Scenes, ect..
Erik Trimble's AD&D Home Page
I Gotta Get Out More!-old D&D and AD&D stuff, including firearms info
DarkSpyre-Forgotten Realms AD&D game, DM utilities, spells, monsters, etc.
Jol Baton Peak-Downloadable adventure modules, charts, tables, and DM utilities
Alan's Dungeon Master Tools-lots of useful stuff for players and DMs.
Ed's D&D page-lots of adventures | The Necronomicon-adventures, etc.
Azumandias' Domain-Misc. AD&D stuff | Paladin's Stronghold-all about holy warriors, and more
 Erik Trimble's AD&D Home Page-rules, races, magic items
has AD&D worlds, games to play in, etc.
Dreamer's Den Role Playing Games-RPG retailer with large selection of imagination games
Games
Irony Games is a great a PBEM site with hundreds of new PBEM games of all sorts in addition to almost everything a PBEM'er could want at a web site.
Nightcrystal: Hope in Darkness-good AD&D PBEM
The Dragon's Cave-links, images, AD&D game | Legends of Vernook-AD&D campaign
1001 Thurian Knights-house rules, PBEM game, and an AD&D world.
Realms Unlimited-AD&D game, etc. | The World of Titan-AD&D campaign world & PBEM
DarkSpyre-Forgotten Realms AD&D game, DM utilities, spells, monsters, etc.
The IRC AD&D homepage-internet relay chat gaming
Dungeons&Dragons online- IRC AD&D game | The Inn of Evermore-AD&D game
Holo-world Magic Realms-a fantasy RPG gaming site (not AD&D)
The Dungeon Dwellers' Guild- fantasy images, an AD&D group
Night Below- an interesting AD&D campaign | The Rjurik Wildlands-a Birthright PBEM game
 Tales from Dunya-PBEM, in Africa-like setting
The PC and PBEM Birthright Campaign Page-PBEM campaign in Cerilia
 Dark Covenant-Ravenloft campaign with new adventures, characters, and other resources.
Antyrrian's Castle of Delights-AD&D game | The Assassins-AD&D campaign
Ollis Adventure Team-PBEM AD&D game
Qrusader's AD&D page-AD&D PBEM | Fallows Cross Adventurers' WWW site
The Dragon's Rest-PBEM AD&D stuff, spelljammer info | The GEAS village-online RPG
Darkstar's Domain-PBEM, stories
DM's Retreat-non-internet AD&D game and other useful stuff
If you want to find an online RPG to play in try 
Black-Dragon-PBEM RPG site which is still pretty new and doesn't have many games.
Other player resources:NASCRAG-Fun, role-playing, puzzles, humor
Unearthed Arcania-home of the new netbook w/new spells, classes, monsters
Qubrak Shata's AD&D page has lots of interesting stuff.
Custom RPG Art-for Gamers; Character Art, Monsters, Scenes, ect..
Alan's Dungeon Master Tools-lots of useful stuff for players and DMs. | The Glass Tower
Saman's AD&D Druid homepage | Mark's AD&D page-links, adventures, kits, etc.
The Howling Caves of Pandemonium-equipment, magic, kits, links
Dragar's Homepage-campaign world Irial, magic stuff, etc.
Erik Trimble's AD&D Home Page-rules, races, magic items
 Blade's Cafe-mostly spells | Paladin's Stronghold-all about holy warriors, and more
Dreamer's Den Role Playing Games-RPG retailer with large selection of imagination games.
Forgotten Realms: Jannes Homepage | Realms Domain
The Realms of the Underdark-Dark Elves and other cool stuff
DarkSpyre-Forgotten Realms AD&D game, DM utilities, spells, monsters, etc.
Fallow's Cross Adventurers WWW Site | Gwineth's Home Page
Yet Another Forgotten Realms Page | Grimspear: City of Wonder
Kalador's Realm | Highmoon-a town| The Righteous Fist | Zhentarim on the Net
Forgotten Realms Campaign Information | Harper's Hill
Dragonplay-misc. stuff, planescape too | Toran's Tavern-Dragonlance stuff too 
Greyhawk: Vibrant Village of Chatwin-an original village in Greyhawk
Gwineth's Home Page
Shouit's Shadowlands Homepage has greyhawk stuff, links, and other stuff.
Hoody's: Underground AD&D Links | Greyhawk Page
Greyhawk:SAGA-greyhawk info for players and DMs, races, maps, etc.
 The Griffin's Crier | Simibnya's Greyhawk page -new spells, monsters, NPCs, adventures
 World of Greyhawk / Jay Hafner
 Links-a page with lots of Greyhawk links
 Cornelius' Greyhawk Page | The Creative World of Gary Gygax
 The Assassins | The Mystical Fantasy World of MERK | Chuck's Greyhawk Campaign
The Shattenjäger's world of Wonder-seller of out-of-print greyhawk stuff
Greyhawk -- Out of Print-info on Greyhawk products
Aloha Greyhawk Greyhawk Grimoire and Heraldry | Mordenkainen's Ultimate Greyhawk Page
Greyhawk Adventures WebRing Homepage | The Company of the Silver Dragon

Mystara:The Estate of Achelos-AD&D game, Mystara info, Mystara Arcade games
Mythblade's Mystara Page-Mystara info&links | Firstmage Home Page
Glenn's homepage-non-mystara stuff too | Dungeon Master Tools | The Principality of Mysteria
Mystara | Mystara (another one) | Mailing Lists (Mystara Archive) | Fabrizio's Homepage
 Dleggen'd Mystara Page-Mystara Maps | Arcanum of Mystara
Geoff's Mystara Site-info on places in Mystara, maps
Dungeons and Dragons-articles from the Mystara Mailing List and other places | Mystara Online
 The Mystara Gazetteer-new&interesting information about Mystara's people, etc.
Fabrizio's Mystara Homepage-synopsis of campaign, etc. | Underocean-small site about sea peoples
Herve's Mystara Page-stuff written for Mystara | Marco's Mystara Homepage
Ravenloft: RealmsOfEvil.com-cool things for evil characters
 Secrets of the Kargatane | The Lonesome Road | Richten's laboratory-Kits & images & stuff
 Dark Covenant-Ravenloft campaign with new adventures, characters, and other resources.
 Crypts of Ravenloft-about novels, games, and downloads | Lord N'astirith's Domain
Dark Sun:
Allied Giants-Dark Sun Links page
 ALLIED GIANTS-guild of Athasian giants in online game
The Land of the Crimson Sun
 Defiler's Acension - A Dark Sun Campaign
 | Gerald's Dark Sun Page
 The Story Of Ore * Dark Sun Online-Download Files, Images And Maps, Text Files, And Links
 Dark Sun Campaigns-description of a game | The Weapons of Tyr page about a gaming group
 Spazz's Home-pictures, characters, campaign ideas, etc. | The Dragon's Sanctum
 Dark Sun Resource Site-Kits, Spells, other Player/DM resources
 Loraque on Athas-short stories | Dark Sun the felixmeister way
  Obsidian Chronicle
A Wrinkle in Time-Dark Sun List FAQ, Dark.Sun.Net.Project "The Champions of Rajaat"
Gundum's Training Pit | The Dead Lands of Athas "Dead Lands of Athas" Net Project, etc.
 The Burnt World of Athas | The City State of Nibenay-rules, magic, psionics, equipment, etc.
 Lawkeeper Ekfenu's Dark Sun Page-short stories, etc.
DragonLance: Fizban's Domain | Eyal's Dragonlance Page
The Realm of Oblivion-AD&D game and adventures too | The Medieval Gigabyte
Toran's Tavern-Forgotten Realms stuff too | Laurana´s Page of Fantasy
The Great Library-heroes and a book list | Kitiaria's World!-book reviews&art
Castle Firestorm-Mostly stuff about magic | The Dragon's Dragonlance Web Page
Tanis' Home in The Qualinesti Woodlands-pics of characters, dragons, etc.
 Dragonmaniac-dragon art | Dragonlance.Com
Enrico's Kingdom | The Ice Lord's Fantasy Art Page | The Dark Tower
Windex's Dragonlance Domain - Solace | Unicorn's Page-about dragonlance books
DragonLance World | Takhisis' Lair | Janne Mi's DragonLance Website
 Dragonlance Castle Bayguard of Palanthus | The Death Knight's Dragonlance Realm
Inn of the Last Home | The High Clerist's Tower -- Dragonlance
 Tasslehoff's Travels | The Lost Citadel | Noob's Official DragonLance Site
Shadoe's Dragonlance Page | Dragonlance: Mount Nevermind
 Kencyclopedia.com-Kender! | Lookas's DragonLance Page | Melissana's DragonLance Page
DragonLance 5^th Age: Dragonlance: Fifth Age
Birthright:  Darkstar's Domain-PBEM, stories
Birthright: Destiny of Regents Online Campaign-good place to find a Birthright game
The Legacy of Kings Homepage-new monsters, awnsheghlien, book reviews etc.
Aiden's Temple of Laerme | Birthright Netbook-original material for Birthright.
Bearcat's Birthright Homepage-PBeM campaign, new rules for Republics, new republic: Alklund
DOR Adurian PBeM-Destiny of Regents PBeM game | DoR Anuire Page | Highlander's Delight
Terence's Birthright Site | The PC and PBEM Birthright Campaign Page-PBEM campaign in Cerilia
Traven's Wildlands-Online Birthright PBeM Campaign set in the Rjuven Wildlands
Destiny of Regents: Rjurik Highlands-PBEM game | The Rjurik Wildlands-a Birthright PBEM game 
Spelljammer: Spike's Hammer | The Mystical Fantasy World of MERK
 Port of Anacostia! | Beyond the Moons-DM stuff, races, classes, skills, monsters, rules, ships, etc.
 Spelljammer | Stormspace - An AD&D Game World | T.O.M.'s Jump Points
 The World of JERA! GM | SpellJammer Crew Cost | Liche Hunt
Planescape: Mimir.net| Realm of Gilheru | Dragonplay-misc. stuff
Zhentil's Tower | Phoenyx Rising | Kristian Planescape | Shivian's Net Archive
The Outlands-Planescape Monsters, free utilities, pictures, and campaign info 
Al-Qadim: No links here as of yet.  Sorry.
Sites With Good Links:
 Woodelf's RPG index | The Legolas RPG AD&D page-Also an AD&D world
Warlocke's page-misc. stuff | Peta's AD&D page-interesting archive of AD&D info too
The AD&D Site Exchange-Well-sorted, quickly growing list of AD&D links
Vlad's AD&D page-netbooks, new classes too | WebRPG-"the online community for gamers"
Arthur Knight's Homepage, consisting of files, classes, a good links page, and much more.
 Juuso Vesanto's Worlds on the Net-links to RPG worlds and other sites (not only AD&D)
Shaolin's web page-AD&D links, info, etc. | Mark's AD&D page-links, adventures, kits, etc.
Rated AD&D Web Sites

Unearthed Arcania Advanced Dungeons and Dragons netbook
The Howling Caves of Pandemonium-equipment, magic, kits, links
-links to (mostly non-AD&D) game sites