when I go to Google and type in "megadungeon", as I'm wont to do every few months, I always find something interesting. Megadungeons are a feast for the imagination.
Underworld Kingdom is making megadungeons by mashing multiple castles together. Very cool!
Blood of Prokopius is playing How to Host a Dungeon, and I do love it when people play How to Host a Dungeon.
Swords and Stichery dug up this historical gem: a propsal to build a 1 million-person city spanning the continent from coast to coast.
Some King's Kent is posting the design of the Aione Megadungeon. Look closely at some of the diagrams, they are spectacular!