GIMP: GNU Image Manipulation Program
GIMP could be considered the poor-man's (or perhaps smart-man's) version of Photoshop, a powerful free-to-use graphics toolset without the hefty Photoshop price tag. Although Microsoft did a fairly decent job updating its native Paint program in Windows 7, it doesn't compare to this graphics editor.
GIMP users can manipulate images via a robust toolbar that offers Photoshop-like abilities. These include the Airbrush Tool, Select by Color Tool, Sheer Tool, and more. The program even has a layer function for rendering multiple effects. This is extremely helpful when combining and editing multiple images in one compilation.
Need to eliminate the barrel distortion of a camera lens? There's a tool for that. Need to fix the image's perspective? There's a tool for that, too. GIMP even supports various devices right out of the "box," including pressure- and tilt-sensitive tablets. Even better, users don't have to spend loads of money for an awesome photo-editing toolset.
Download: GIMP v2.6.10