GIMP

GIMP:  is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo editing and image composition  It works on many operating systems and in many languages.

This is a screenshot of how the program is designed.

If you want to download it here is the link to the official downloading webpage.
http://www.gimp.org/downloads/ 

A feature I really like about this is how you can share and help with other users of GIMP making it a nice sociable network to work with.