Back around mid-2012 I decided to create a Master List of D&D Retroclones and Simulators. For a time it became a very popular resource before it was eventually eclipsed by Taxidermic Owlbear's own list (which was inspired in part by my previous efforts).
In regards to last year we saw the release of several mega-dungeons. The long-awaited Dwimmermount, Castle of the Mad Archmage (a sort of inspiration of the old Castle Greyhawk legacy), and soon an upcoming Barrowmaze Complete!
I figured that a compilation of megadungeons would be a worthy project. But first we should define what a megadungeon is. My terms are broad: a megadungeon should be a large enough dungeon which could encompass the better part of a campaign for players to explore session after session. Note that while some megadungeons might be spread over multiple products (some of them being more recent Editions), I intend to cover megadungeons built for OSR games or whose legacy is encased with the TSR eras of D&D for the interests of this forum.
For those wishing to build their own megadungeons, this thread contains some very useful advice on the matter.
So here's my list so far. Tell me if you have any others to add, or if I missed any relevant products:
Anomalous Subsurface Environment (ASE1 and ASE2-3, available for Labyrinth Lord)
Barrowmaze ((I & II, available for Labyrinth Lord)
Castle Greyhawk (WG7 Castle Greyhawk for 1st Edition AD&D, Expedition to the Ruins of Greyhawk for 3rd Edition D&D, Castle Zagyg series for Castles & Crusades)
Castle of the Mad Archmage Adventure Book (available for Adventures Dark & Deep and Pathfinder)
Castle Triskelion (available for AD&D 1st Edition)
Dwimmermount (available for ACKS and Labyrinth Lord)
Rappan Athuk (available for Swords & Wizardry and Pathfinder)
Stonehell Dungeon (available for Labyrinth Lord, Down Night-Haunted Halls with 2 bonus Supplements)
Temple of Elemental Evil (available for 1st Edition AD&D, has sequel adventure Return to the Temple of Elemental Evil for 3rd Edition D&D)
Undermountain (Undermountain I and II for 2nd Edition D&D, Ruins of Undermountain boxed set, Expedition to Undermountain for 3rd Edition D&D)
One of my readers suggested:
Greyhawk ruins for 2nd Ed
Castle Whiterock from the d20 DCC line