Here are some more.
Great program for serious application or just fooling around
Excellent for creating seamless textures from ANY image
Free - useful for opening DEM (Digital Elevation) files
Photoshop plugins. India ink and Lunar cell are pretty cool
The heavy weights ...
The Daddy ...
Excellent! good for ecosystems - massive learning curve but worth it
Pain to use ... has some cool features such as importing OSM (Open Street Map) data