Most times, I've simply used Photoshop or some other graphics suite/program to create or edit maps, myself. If you're leaning in that direction and want something "worth while" on the cheap (that is to say "free"), I'd recommend
the GIMP... free, open-source, image editing... with a few plug ins and a bit of practice, you can make this thing do everything that Photoshop can, I think.
Now, as for actual map making software, there is a program out there called Campaign Cartographer... not free but definitely a very cool product... if I had need of such things more often, I'd definitely invest, myself.