GIMP (GNU Image Manipulation Program) is a free, powerful alternative to Photoshop. It is easy to use with a range of features for casual to professional users. It is an especially great choice for those with a low budget, or those looking to get into photo editing.
Pro Free, open source and cross-platform
GIMP is completely free and open source, meaning you can use GIMP and all of its features without spending a penny. This makes it an excellent case for artists or designers who may not have the budget to spend over 700 USD on Photoshop. It is also available for free on Windows, Mac OS and Linux. It's licensed under GPL with source code available here.