I like MapTool. I've also been known to use the GIMP to tile together images, but that's only worthwhile if you've already got tiles.

I've actually used the toolkit from the original Neverwinter Nights to generate a map, but this is rather slow and whatnot.

I've also messed around with other things but they're mostly random in nature, so I'm assuming you don't want them. If you are, I'd suggest http://donjon.bin.sh.

Also, I just found an awesome site for this: http://pyromancers.com/dungeon-painter-online/

Hopefully one of these is helpful.

I like MapTool. I've also been known to use the GIMP to tile together images, but that's only worthwhile if you've already got tiles.

I've actually used the toolkit from the original Neverwinter Nights to generate a map, but this is rather slow and whatnot.

I've also messed around with other things but they're mostly random in nature, so I'm assuming you don't want them. If you are, I'd suggest http://donjon.bin.sh.

Also, I just found an awesome site for this: http://pyromancers.com/dungeon-painter-online/

Hopefully one of these is helpful.

Edit: Dunjinni looks like it could potentially be cool, but I'm not very familiar with it. There's also Dungeon Maker, which is less fully featured than Dungeon Painter Online but a good backup just in case, Tiamat, which falls somewhere in-between, and PyMapper, which is downloadable, though I had issues with its installer (and I don't have Python on this computer to test the from-source version, though if you had Python you could click-to-run it).