Gimp is free then there is Photoshop. There are loads of non free alternatives such as Serif plus, Xara etc and you can look down the software list in the forums here. Also, look through the final maps sections and look for maps you like and read off of the thread what software was used.

If you need to print maps then you can either chop them into A4 and print - PosteRazor is a good free app for that. Also, there is the member Gamerprinter here who runs a print shop specifically tailored to printing RPG maps with options to laminate etc.