1. GIMP
GIMP is one of the best known alternatives to Photoshop, and the layout is strikingly similar.
2. Krita
Winner of the Akademy Award for Best Application in 2006, Krita is easy to use and has some unique features.
3. Paint.NET
Free image editing software for Windows, Paint.NET supports layers, unlimited edit undo, and a variety of special effects and handy tools. An added bonus is the growing Paint.NET community and tutorials for any query you could possibly imagine.
4. Cinepaint
Although Cinepaint is primarily used for making animated feature films, it is also a fantastic image editor which supports 32 bit colour channels and 16-bit TIFF files.
5. Pixia
This Windows application made by the Japanese specifically for anime, has been translated into English, and there are many tutorials floating about.
6. Pixen
This Mac application is best for animation and incorporates layers for efficient editing.
7. Picnik
This free photo editor has all the basic features, as well as advanced ones such as layers and special effects. Because this is a web based application, all you need is access to the Internet, and it runs on Mac, Windows and even Linux.
8. Splashup
This web based application closely resembles Photoshop. Functionality includes layers and the ability to edit several images simultaneously. Splashup integrates beautifully with all the major photo sharing sites, such as Facebook, Picasa and Flickr.
Although this Abobe package is free, it does lack one crutial ingredient – Layers. This cross platform application also intergrates beautifully with most major photo sharing websites.
10. Aviary
Phoenix is a free Image Editor from Aviary. A comprehecsive toolset allows you to undo, redo, use brushes, the magic wand, and a variety of blend modes, as well as work with layers and an intuitive drag & drop interface. Collaberate with other users and follow step-by-step tutorials to improve your skills. Aviary also has a host of other impressive applications, including an Effects Editor and Vector Editor.