We may no longer have megadungeon.net (not linked, because now it's a club promotion site), but now we may have something better.
I've been getting a number of hits from http://www.megadungeons.com/, a site that:
"Hope to create a reasonably exhaustive list of megadungeon resources, categorized in a way that I at least find useful as I work. In addition to the Resources page, I’ll be blogging about my learning-by-doing experience of Knightley Keep, any resources I create myself, and anything else that goes down in the wide world of megadungeon creation."He links several of my resources, which are great for megadungeon design, but strangely doesn't mention my own megadungeon, Numenhalla.
Even though the site is new, the resource page is robust. There's a single forum for mapping requiring a wordpress log in. It's worth dropping by and saying hello!