Adobe Photoshop Express
This express version of Photoshop allows users to quickly edit images online. Photoshop Express is easy to use, professional and gives you 2 gigabytes of photo storage for free.
Gimp or GimpShop
Gimp is the most popular Photoshop alternative, with many tutorials available. GimpShop is a special version of Gimp made to resemble Photoshop.
SumoPaint allows users to edit images or create designs from the Web. SumoPaint's features include layer effects and the ability to use fonts from your computer.
Phoenix is part of the Internet design suite named Aviary. Phoenix provides popular Photoshop features such as alpha channels and masking abilities.
SplashUp allows users to edit images from websites such as Flickr and Facebook. It's user-friendly and features a simple editing process.
Paint.Net is for those interested in the drawing features on Photoshop. A great feature of Paint.Net is its unlimited "undo" option.