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.