Gimp is an open-source alternative for Photoshop. It has all basic picture editing tools and many advanced features. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. It is compatible with most operating systems (Windows, Linux, MacOS X …).
Gimp has many tools to manage color saturation, brightness, contrast … The picture bellow presents the GIMP user interface and how to adjust a picture color curve.
In this portrait image I made some other changes in the background of the picture like hiding some part and applying a blur filter. There is a lot of free tutorials about Gimp. To make those changes I used this link : http://gimpguru.org/tutorials
I think GIMP is very good tool to edit and enhance pictures. I find it better than pictures editing software provided with my camera which are a little bit basic and which change from one constructor to another.