Gimp (GNU Image Manipulation Program) is a free, open source image editing tool, similar to photo shop.
Since it's free, you would think the user interface would be a bit rough around the edges. Instead, the program is clean, easy to use and full of powerful tools! I have limited experience editing photos, but was able to figure the basics out fast.
It has everything you would expect and need to edit photos on an amateur or professional level. Features include the standards like layers, channels, cloning, filters and effects. There's many more tools available and plenty of tutorials to follow.
At first, my thoughts were that this software is probably going to have some major flaw, but the volunteers have created a true gem in GIMP.
free GIMP software download
gimp is available for the following operating systems: Linux/GNU, Mac, Windows, and Solaris!