Yesterday Dyson Logos and Mike from Gutter Cult pointed out what is perhaps the end-all-be-all of the online dungeon generators: Dave's Mapper.
It uses geomorphs from a ton of sites (see here) and let's you pick and choose those you want to include in the generation. It also includes options for city or village generation. Very cool.
They *only* thing I see some of the other ones I posted having over this one is that they provide monsters, traps, etc. However, if you just want a spiffy looking map to populate, Dave's is the place.
On a side note, thanks to Kevin for sharing his Random City Intersection generator. That could be handy for modern/super campaigns.