This list was created by Nat of Megadungeons.com. As Nat has taken Megadungeons.com offline, I hope he doesn’t mind me making this available here.
I have made a few updates and additions and will continue to edit and improve this list. Please contribute in the comments if you can.
The original campaign dungeon, designed and run for decades by Gary Gygax himself along with Rob Kuntz. Most of the dungeon was essentially unpublishable, as it was in the form of Gygax’s own terse notes and maps, but attempts have been made over the years, with and without Gygax’s involvement. TSR published a Castle Greyhawk module in 1988, but rather than being based on any of Gygax’s work, it’s a collection of jokey theme levels that are generally seen as a deliberate insult to Gygax after his departure from TSR. Naturally it is widely reviled. TSR made a more serious attempt in 1990 with Greyhawk Ruins for AD&D 2e; this module was updated in 2007 as Expedition to the Ruins of Greyhawk for D&D 3.5. Starting in 2005, Gygax began publishing pieces of the castle through Troll Lord Games for their Castles & Crusades system. The Castle Zagyg modules (renamed since Wizards of the Coast owns the name “Greyhawk”) cover the Free Town of Yggsburgh (i.e. the Free City of Greyhawk) and the “Upper Works” of the castle, plus a few other areas. These are now out of print, as Troll Lord lost the license after Gygax’s death, so finding them is up to you. Refer to this list of the Castle Zagyg products at RPG.net. Readers curious about the complex history of the ultimate megadungeon, and what is known about Gygax’s version, are referred to Allan Grohe’s exhaustive Greyhawk Castle Archive.
The megadungeon of the Forgotten Realms, Undermountain has seen many published versions. Unlike Gygax, Undermountain’s designer (and Forgotten Realms creator) Ed Greenwood never ceased his working relationship with TSR/WotC. Thus Undermountain is the signature megadungeon of official D&D. Published versions include 2012′s Halls of Undermountain for D&D 4e, Expedition to Undermountain for D&D 3.5, and for AD&D 2e, the Ruins of Undermountain boxed set, The Ruins of Undermountain II: The Deep Levels, and the Dungeon Crawl trilogy of the Lost Level, Stardock, and Maddgoth’s Castle. (All of these can be found used online. The original boxed set is highly recommended.) The dungeon has also seen play in video games, including the reviled Descent to Undermountain and the acclaimed Neverwinter Nights.
The brainchild of James Maliszewski of Grognardia, Dwimmermount raised nearly $50,000 on Kickstarter from over a thousand backers. Not sure how it’ll be made available to non-backers, but you can rest assured JM will have updates at Grognardia. You can see the Kickstarter page here.
Created by Greg Gillespie, Barrowmaze is available in pdf, softcover, and hardcover formats at rpgnow.com. You can also buy hi-res maps separately in either black-and-white or awesomely old-timey blue-and-white.
Michael Curtis’s Stonehell Dungeon was first released in 2009. It’s available in pdf and softcover at Lulu along with two supplements that expand the dungeon.
CASTLE OF THE MAD ARCHMAGE
Few know Greyhawk better than Joseph Bloch of Greyhawk Grognard (a great gaming blog in its own right). Castle of the Mad Archmage is his take on the granddaddy of all megadungeons, Gary Gygax’s Castle Greyhawk. CMA spans some twelve levels of old-school dungeony goodness, has been hailed as one of the best products of the old-school renaissance, and is available for free in pdf form (though word is that it’s going to be officially published by Black Blade). You can also nab hi-res poster maps.
Bill Webb’s monstrous Rappan Athuk has gone through two versions already: the original Rappan Athuk, and Rappan Athuk Reloaded, both released in the early 2000′s for 3rd edition D&D. The recent Kickstarter to produce a new version of RA (statted for both Pathfinder and Swords & Wizardry) raised an incredible $246,541 to release an even bigger Rappan Athuk through Frog God Games (who swallowed up RA’s original publisher, Necromancer).
Anomalous Subsurface Environment
ASE1, the first part of a megadungeon by Patrick Wetmore (known as PatW on various forums), includes a long description of the history and situation of his dungeon, as well as the first level. ASE is known for its gonzo attitude; set in a future Earth, it includes dinosaurs, super-science, and all sorts of whatnot.
The in-house megadungeon of the Hack & Slash blog, Numenhalla is built around the credo “Don’t be a boring person telling about boring things.” It’s rare to see a megadungeon spending so much time on setting, but the Numenhalla posts spend as much or more time on the gods, races, etc. of the world as on the dungeon itself.
World’s Largest Dungeon
Published by AEG in 2004, the World’s Largest Dungeon weighs in at over 800 pages and at the time, was both the largest and most expensive roleplaying book ever published. Containing every monster included in the d20 SRD, the dungeon is designed to take characters from level 1 to 20 and includes over 1600 rooms. There doesn’t seem to be much support for it any more, but you can still buy WLD on Amazon.
Maure Castle was a location used in a campaign that Robert J. Kuntz ran for Gary Gygax, and they later both worked on. This campaign, in its pre-commercial form in 1972-1973, was the seed from which much of the World of Greyhawk campaign setting and the Dungeons & Dragons game itself originated.
The Dungeons of Castle Blackmoor details the 20 known levels of the dungeon including the first 10 levels as originally created by Dave Arneson himself. Designed to scale with varying party sizes and power levels, this incredible dungeon includes adventure material for character levels 1-20.
Goodman Games Castle Whiterock is a complete campaign. designed to take new characters to 15th level and fuel a game for years.
Caverns of Thracia
Caverns of Thracia is a classic Judges Guild dungeon which was expanded and updated for 3.5 by Necromancer Games.
Tegel Manor is a 240 room Judges Guild dungeon which has been republished by Gamscience.