Mmm.. Kinda cool. This is a resource worthy of being stuck to the top of the forums, me thinks.
Actually, I was playing around with Microsoft Visio today and I noticed it had a map builder built in as well. I need to play with it more, but it might just be an easy wasy to whip up dungeons in the future. But, it didn't have graphical tilesets like this online tool from Wizards.