From the Paint.net website,
Paint.NET is image and photo manipulation software designed to be used on computers that run Windows XP or 2000. Paint.NET is jointly developed at Washington State University with additional help from Microsoft, and is meant to be a free replacement for the MS Paint software that comes with all Windows operating systems. The programming language used to create Paint.NET is C#, with GDI+ extensions. Paint.NET has many of the powerful features that expensive commercial applications have, including the ability to use layers.This is a screenshot from their website:
Paint.net - Screenshot
And, in case you are wondering, the source is freely available for download!