I've been asked if there's anything that can help with game mapping that I'm aware of, and so I've had to do a bit of research to answer the question.
The first thing that came to mind was OmniGraffle for some reason, but a little research quickly turned up Gliffy which offered something very similar (free basic account available). Some version of D&D was supposed to include a CD-Rom with map making software, although all of the links to it I can now find are dead.
Profantasy has some powerful looking software, but it's about $50.
Dunjinni looks great too, and is slightly less expensive, if you don't mind paying for this type of thing.
Inkscape has been suggested by some, which I have used for other purposes. There's always the pen and paper method of legend!