Paint.NET is free image and photo editing software for computers that run Windows. It features an intuitive and innovative  user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. An active and growing online community provides friendly help, tutorials, and plugins.
 
It started development as an undergraduate college senior design project mentored by Microsoft, and is currently being maintained by some of the alumni that originally worked on it. Originally intended as a free replacement for the MS Paint software that comes with Windows, it has grown into a powerful yet simple tool for photo and image editing.