Someone left a cool picture of Mars on my desk at work and as I looked at the image I thought it would be cool to make one in the Gimp, because I typically use brushes or actual planet pictures. Amazingly it took me only 5 minutes and really requires no real skill in Gimp or Photoshop so I decided everything in the file itself should be the tutorial. It's pretty self explanatory, even for someone fairly new. Obviously if you do not understand layers you may need to get Grokking The Gimp , which is free and a great starter. The Gimp is also free and can be found here and runs on Windows and OS X, as well as Linux\Unix.

The tutorial is of a simple moon, which contains three layers, a circle selection filed with a light white to black gradient, a texture for the surface, and a second circle selection with a darker white to black gradient. The file can also be used as a template, add your own texture(s) \ colors and play with the layer settings, I have it set to "overlay". 

Download the template, around 1.6mb (includes both Gimp XCF and Photoshop PSD formats)