I have seen some awesome megadungeons from various people in the old school community over the years -- bits of Allan Grohe's Castle Greyhawk, Stefan Poag's Mines of Khunmar, some unpublished parts of Bill Webb's Rappan Athuk, and even those couple of photos of the real Castle Greyhawk that were visible from behind Gary at some convention or other.
However, when I was DMing at NTRPGCon, someone showed me the map of one that just blew me away with its ... I don't know ... epic quality is I guess the best description of it. If you happen to run into Austinjimm at a convention, ask for a glimpse of his dungeon, because it will take your breath away.
His blog is here, although I don't think there are any pictures of the actual dungeon in there.