Open source version of Microsoft Paint
Graphics programs are very complex these days, but OpenPaint takes things back to the early days. It's an open source version of Microsoft Paint, the simple program that comes with Windows.
Still in beta, OpenPaint suffers from a little instability, but is otherwise functional. The tools are basic - you get a brush and pencil with no settings, the fill, spray and basic shape tools and a few more.
With no line tool, OpenPaint is actually more basic than Microsoft's version. There isn't really anything to help you create images - you need steady mouse control, and have to accept this app won't be able to create very subtle images. OpenPaint's instabilities are more annoying - occasionally when you use the shape tool, your shape might disappear after you've drawn it.
As Microsoft Paint still ships with Windows, there's little reason to use OpenPaint which offers no advantages, and isn't improved with it's extreme simplicity!