Qaquarelle is the opensource Qt4-based graphical editor, whose goal is to provide the native way of painting with emulated traditional instruments, including the full support of tablet input and OpenGL-based processing in GPU.
Pixi image editor. Common features like: layers, im/export bmp/png/jpg/tga/tiff, scripted command interface, alpha, layers are worked on as floating points. Written using freepascal, and a work in progress. Modular design. Latest source in HG.
UberPaint (formerly 4P) is a pixel pushing paint program heavily based on Electronic Arts' "Deluxe Paint". It tries to replicate the behaviors and functionality of DPaint 2, while adding support for more file formats and modern computer OSes.