GIMP - Image Manipulation Software
GIMP is a free, open source photo editing software.
Digital Artwork From Scratch In GIMP
Plugin Registry that supports rare file extensions
GIMP includes a very unique support for various input devices out of the box. Pressure and tilt sensitive tablets, but also a wide range of USB or MIDI controllers. You can bind often-used actions to device events such as rotating a USB wheel or moving a MIDI controller's slider. Change the size, angle or opacity of a brush while you paint, bind your favorite scripts to buttons.
GNU/Linux (i386, PPC)
Microsoft Windows (XP, Vista)
Mac OS X
GIMP Running In Windows
GIMP Running in Linux
If you like GIMP you can also check out GIMP Shop which is designed to feel more like photoshop.
GIMPshop is a modification of the free/open source GNU Image Manipulation Program (GIMP), intended to replicate the feel of Adobe Photoshop. Its primary purpose is to make users of Photoshop feel comfortable using GIMP.
Download GIMP Shop
GIMPshop modifies the menu structure to closely match Photoshop's, adjusts the program's terminology to match Adobe's, and, in the Windows version, uses a plugin called 'Deweirdifier' to combine the application's numerous windows. While GIMPshop does not support Photoshop plugins, all GIMP's own plugins, filters, brushes, etc. remain available.