I use Photoshop almost exclusively. I've fiddled around with Terragen but the output image is always messed up in a top-down view (anomalies around the edges, particle interference from ambiance washing out the colors etc). Also with Bryce but I mess things up quite a bit and Bryce is not very flexible for that (steep learning curve). I've tried every free art, 3d, and map or world making program out there but I always come back to Photoshop. Granted there are many things it can not do (colored brushes or painting with a pattern or properly rotating a brush while stroking a path or having objects cast shadows instead of simply plopping down a floating dark space, etc). So I just work around these limitations, which takes ten times longer but gives more control in the end anyway.