GIMP 2.0, you can make brushes that are mountains, trees, etc.
Or, you can use Cartographer. It is a paid version, however, you should be able to find it for free somewhere. It takes a bit to get used to, but it works.
GIMP works well though, it is just a matter of looking up tutorials on how to make a map. Theres some that will tell you how to make brushes and how to lay them out. Just remember: Start your mountains fromt he top left and drag them down from there. Makes them look a lot better.
If you need more info, shoot me a PM, I have some stuff you can look at.
Happy Map Making!