I just found this while looking for art software, I haven't actually heard of it before so I thought I would share the info.
GrafX2 is a bitmap paint program inspired by the Amiga programs Deluxe Paint and Brilliance. Specialized in 256-color drawing, it includes a very large number of tools and effects that make it particularly suitable for pixel art, game graphics, and generally any detailed graphics painted with a mouse. The program is mostly developed on Linux and Windows, but is also portable on many other platforms.