Simple to use paint program like Paintbrush for Mac or classic MS Paint, but for Linux.
Pinta is a graphics drawing/editing program modeled after Paint.NET. It strives to be simple yet feature-rich, and runs cross-platform. Drawing tools include freehand, lines, rectangles, ellipses, and more. It allows unlimited layering, undo and redo history, over 35 image effects, and supports floating or MDI workspace layout.