GIMP is a really great program for being an open-source thing. Just a word of caution, though... Once your files get past about 20 layers and a few thousand pixels in either direction, the program becomes almost unrunable. My recent Nesspressen map ended up being 6,900 x 5,400ish with about 40 layers, and every action I did took at least five minutes on a dual-core laptop with 2.5 gig of ram.

Have fun, though, as GIMP can do a whole lot!