Great option to other image editors and image creating software. It has a simple and intuitive user interface, yet supports both layers and 3D objects. Functional across all major OS. It is free, as in open source. It imports .PDN files created with Paint.NET without issues.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
miniPaint - Online graphics editing tool use HTML5 technologies like Canvas, Offline support, Drag and Drop.
GIMP - GIMP is a multiplatform photo manipulation tool.
RealWorld Paint - RealWorld Paint is an free portable image/photo/animation editor.
Microsoft Paint - Paint (formerly Paintbrush) is a simple, ultra-basic graphics painting program that is included...
Paint XP for Windows 7 - Standalone old MS Paint program that allows you to get your old XP Paint interface in Windows 7.