Pixlr.com is a great online alternative to Photoshop. Nothing to download or install. It all lives on the web and is accessed through your flash-enabled web browser! If you are used to how Adobe Photoshop functions, you will be at home with the advanced Pixlr editor. It looks amazingly a lot like photoshop, and even many of the menu structures and options are set up the same. Complete with layer effects, masks, and more, if you need a FREE Photoshop alternative, give Pixlr.com a try!
Believe it or not, most commercially available software has a FREE twin! (Or at least distant cousin!)