(This item is crossposted from theRPGsite.com forums.)
If old school D&D is your thing, I heartily recommend Dragonsfoot as the premier source for such things. The forums there are fantastic, marred occasionally by anti-d20 whining, but excellent nonetheless. In the middle of a lengthy thread about megadungeon construction I found the single best analysis of dungeon map design I have ever seen:
If you ever do dungeon crawls in your games, do yourself a favor and follow that link.
(TheRPGsite.com discussion here.)