The Dungeon of Doom was the centerpiece megadungeon of my mid-to-late 90's AD&D campaign set in the Bandit Kingdoms of Greyhawk. I've run more forays into this sadly-misplaced dungeon than any other D&D environment save the Caves of Chaos in module B2 The Keep on the Borderlands.
Much of the dungeon was generated using the random charts in the back of the first edition DMG, with one slight variation. When a monster was called for I would roll a d6. On a 1 or 2 I would then use the Wandering Monster charts from the DMG to determine which critter to use. On a 3 or 4 I would go with the Fiend Folio charts and on a 5 or 6 I'd use the Monster Manual II tables. The result was that the monster mix was skewed heavily towards the original Monster Manual, but with a good smattering of weirdoes from the other two books. Also, I used whatever creature I rolled, no matter how clumsy the mechanics or how little I like the monster. If the dice said the room had 2 vulchlings, then by Cuthbert there were two vulchlings in that room.
As I mentioned, I can't find my notes for the Dungeon of Doom and I fear they were accidentally discarded the last time I moved. But I wanted to share some of my favorite bits. Some of these items are the direct result of following Gary's DMG charts, others are my own invention.
Level One of the dungeon had a 10' x 10' room that was actually an automatic express elevator down to level 4. Once down there the elevator didn't come back up for six hours. In its first inadvertent use I described the sound of the elevator operation as "chunga-chunga-chunga". The players grew to associate that sound with deadly peril. I don't think any of my players ever discovered the Lower Hellevator, which led from level six to level ten.
This campaign was the first time most of the players in my group ever found a sub-level. Of course they did it the hard way. After a bad go on level three they were desperate to get out of the dungeon. The PCs rejoiced when they found a new staircase that went up a level. Hours later they finally figured out that this particular level two had no access to level one. Exhausted and lost, they were forced to make camp on an uncleared level. They survived a night in the dungeon only by sharing a room with a large band of gnomes that showed up on the wandering monster charts.
The Iron Frogs
Instead of the standard pirate booty box, about one treasure chest in six was a hollow iron statue of a frog with a hinged mouth. No particular reason. I just like frogs. The players were mystified.
The Chapel of Evil
Not far from the Hellevator exit on level 4 was the Chapel of Evil, a little subsection of the level kinda like the Shrine of Evil Chaos in module B2. The big difference: the head cleric was a vampire. After a rough initial encounter with the Vampire Evil High Priest and his minions the party withdrew. They came back with more henchmen (including an ogre trained in the art of sumo who was also an expert with the harpoon, IIRC) and more one-shot magic and wasted the place in an epic battle. Good times.
The Baleful Stone
No player ever discovered this secret, but the dungeon had an off switch. According to my sketchy notes for the bottom level (nothing below level 8 ever had a proper key and map) the lynchpin of the dungeon was the legendary White Stone of MWOWM, which I totally stole from the Book of the Subgenius (or maybe its sequel, Revelation X). The evil vibrations from this artifact were what drew so many demons, devils, undead, etc. to the dungeon. If the White Stone were removed from the dungeon most of those monsters would leave and eventually only a mindless slimes and vermin would remain to haunt the rooms and passageways. I always imagined that if the players got their hands on the Stone they would try to give it to the elves without them knowing what it was. They were that kind of group.
On level seven there was a deep pit that led to the level ten lair of a Pit Fiend. Because I am that literal-minded sometimes.
One of Each
Also on level seven, not far from the Pit, was the chamber that served as the rumpus room for one of each type of demon from I to VI. According to the Monster Manual that mix is a legit possibility when encountering demons. I don't particularly like running demons because of all their fiddly special abilities, but I wanted to see the players react to such a grouping. Across the hall was an enclave of mind flayers driven mad by the White Stone. That's right insane mind flayers.
It was scouting level seven that convinced the players it was time for the campaign to shift focus to wilderness exploration and stronghold-building.