The GIMP 2

The GNU Image Manipulation Program, or GIMP, is a raster graphics editor used to process digital graphics and photographs. It is often seen as a free replacement software for Adobe Photoshop, and thus comparisons between the two is widely debated between users around the world. The program has similar functionality as Photoshop, although it lacks some of Photoshop's features and vice versa. The program can be used to create graphics and logos, resize and crop photos, alter colors, combine multiple images, remove unwanted image features, and convert between different image formats.

For more information, please click the following link:
http://en.wikipedia.org/wiki/GIMP
To download the program, please click the following link:
http://www.gimp.org/

I personally use the GIMP. At first the user interface may seem intimidating, but once you know how to use it, you'll find it is a very powerful program indeed.