PicPick is an all-in-one program that provides a full-featured screen capture tool, an intuitive image editor, a color picker, a color palette, a pixel- ruler, a protractor, a crosshair and even a whiteboard.
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.