The free image editor GIMP but with a Photoshop feel
The GIMP image editor is very powerful but the interface can be a little challenging. GIMPShop is a version of the GIMP that has been modified to work more like Adobe Photoshop.
Many of these changes relate to the structure and layout of menus. The Windows version also uses the Deweirdifyer Plugin so that all the windows appear in one window, rather than the many windows found in the ordinary version of GIMP.
There are also versions of GIMPShop available for Mac OS X and Linux. As it is open source, the source code is also available.