Pixen: Pushing Pixels To The Limit Now in its third iteration, Pixen is a pretty mature graphics package that somehow I've never noticed before.  Pixen is an open-source graphics editor designed for pixel artists--"people who make low-resolution raster art."  This may be true of its initial incarnation, but it's also described as "a very powerful MSPaint or a simpler, more agile Photoshop."  Pixen handles layers and can also do animations that you can export as animated GIFs or QuickTime movies.  Definitely worth trying out!