Introducing the Mac version of Gimp. This is a native Mac version of Gimp that works just like any other Mac application. You need Snow Leopard or higher to use this version. There have been reports that it works fine on Mountain Lion.
Seashore is an open source image editor for Cocoa.
Seashore is an open source image editor for Mac OS X's Cocoa framework. It features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers and alpha channel editing. It is based around the GIMP's technology and uses the same native file format.