I've been poking around the Internet for a couple of months trying to find some basic resources for running an online game, and I'm amazed at some of the maps you folks have created. I'm hopeful that between the tutorials and the forums I'll be able to find what I'm looking for, there seem to be a plethora of information.
Right now I'm mainly looking for a very basic and most importantly an easy-to-use software for creating dungeon maps and building layouts. I have found the thread on the different software and will try some of those that I hadn't heard of before out.
I'm sure if you check out some of the awesome work here and look at what software they used to create it you will find what you need.
I'm using GIMP personally, it's free and makes some beautiful maps from what I have seen. I also have Inkscape which is also free. I like the artistic side of GIMP, and Inkscape has it's own areas of greatness as well (I'm not as familiar with it yet).
There is a thread here where RobA rocks out how to use a script to make dungeons. You might check it out if you haven't already. It's very cool.
I'd recomend you have a look at Campaign Cartographer 3, by ProFantasy:
It's a fairly simple program, and there are lots of tutorials on UTube to help you out.
I use it exclusively, but then, I'm not talented enough to use something like photoshop and create it all from scratch.
Dundjinni is also another option you may want to check into.
I'm going to second Steel General. CC3 is great for overland maps, but you really need to buy at least one add-on to make it good for dungeons and buildings. Dundjinni, on the other hand, is designed primarily for dungeon mapping (thus the name), thus being a somewhat cheaper buy-in if dungeons is all you're going to do.
You might want to try out the Gimp and Inkscape first, though, since both are free. If you find one or both worthwhile, then you can spend your money on a drawing tablet instead of software.
Any which way you go, though, you can find plenty of help with software, theory, and art here.