If you don't mind spending some cash, Campaign Cartographer will help you make rather professional looking maps. Price starts at $45, and there are lots of add ons/plug ins.
EDIT: I haven't used it in a long time, but it had a severe learning curve back in the beginning. I would hope it's a tad more user friendly now.
I use Pyromancers for quickly making simple maps.
There's also Campaign Cartographer if you have a lot of money you don't want any more.
Campaign Cartographer 3, it's a map building program. I'm still getting used to many of the features, but it has some really cool stuff that you can do with it.
Actually, is there even a way to put maps from other programs into GIMP?
E.g. I have used Campaign Cartographer 3 before, but the only feature I like is how you can draw the shape of the map, then it will automatically "roughen up". Could I draw the shape in CC3, then bring it into GIMP?
It's still alive Campaign Cartographer 3. I never liked how much it costs, though its been years since I've looked (i last remember them being on CC2). As a note, its an oddity for role playing utility programs, simply because you could buy this off a shelf at a store.