Paint.NET is image and photo manipulation software designed to be used on computers that run XP, Server 2003 or Vista. It started development at Washington State University as an undergraduate senior design project mentored by Microsoft, and is currently being maintained by some of the alumni that originally worked on it. It is meant to be a free replacement for the MS Paint software that comes with all Windows operating systems.
 

Paint.NET has many of the powerful features that expensive commercial applications have, including the ability to use layers. Requires the .NET Framework 2 in order to run.

Paint.Net was open source until version 3.2, but has been closed since then. This is the last open source version.