Dragonsfoot (www.dragonsfoot.org) has been mentioned here before on Cartographersguild. Its a massive site that focuses on pre 3rd edition versions of D&D (OD&D, 1st Ed., 2nd Ed., etc.). Castlewalls posted here about its cartography section (http://www.dragonsfoot.org/fe/#FEFP17 and scroll down to near the bottom) but its messageboards are what I'm here to point out. Buried within those messageboards are threads that pop up from time to time that are of special interest to mappers and map enthusiasists. There are three you should check out:
The first links to Bargle the Infamous' quick and dirty technique for making oldschool blue ink cave maps at http://bargle.bravehost.com/. If you have some familiarity with GIMP, give it a look. Even if you don't, the free 5 meg pdf with 201! gridded cave maps is definitely worth a snag.
The second is the incredible Megadungeon thread (19 pages of posts and going!) that talks about the design and philosophy behind Megadungeons (multi level dungeons with large numbers of rooms--think Undermountain, Castle Greyhawk, Castle Whiterock, etc.). A poster by the name of Melan, inspired by the thread, posted his maps of the dungeons underneath his City of Khosura here: http://www.dragonsfoot.org/forums/vi...=asc&start=225
(Scroll down past the halfway point on the page to see it). The maps are hand drawn and do not have a grid, but they are very sharp and brought back many oldschool dungeoncrawl memories for me.
Last, but definitely not least, is the collaborative Mapping the Underdark Project found at: http://www.dragonsfoot.org/forums/vi...er=asc&start=0
The project seeks to map out many of the encounter areas shown on the Underdark Regional map in D1-2 Descent into the Depth of the Earth but never detailed in the module. The latest pdf compilation of encounters and maps can be found at http://dead.greyhawk.googlepages.com/mappingthedepths
It should be noted that the encounters are statted for 1st Ed. D&D, so some work may be required converting them to another edition.