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.
Gobby is a free collaborative editor based on libobby, a library which provides synced document buffers. It supports multiple documents in one session and a multi-user chat. It runs on Microsoft Windows, Mac OS X, Linux and other Unix-like platforms.