GIMP is everything
GIMP: GNU Image Manipulation Program
As its name stats, GIMP is a free software for image editing and retouching.
To obtain GIMP, visit http://gimp.org and follow instructions written over there. Do not forget to install gtk before installing GIMP as gtk is required to install and run this software. It is available on Linux and Windows platform.
What attracts me towards GIMP was its necessity. I am an amateur photographer snapping many of the photographs in different situations. My 20-30% of images need calibration or post processing and GIMP solved each and every problem. Now, I am also doing workshops for other photographers using GIMP.
I am preparing to post some of the tutorials for you to start with this free and versatile software.