I've seen some DMs use Excel spreadsheets for maps, alternatively. But of the methods outlined, Maptool really is the easiest to work with, as veekie said, due to it being specialized for the task. Gimp also works. Paint is a nightmare, compared to everything else listed so far, and I recommend against it. All of the methods suggested so far are also free, so there's really no reason to use it (MS Excel isn't free, but the OpenOffice counterpart is and works just fine).