GIMP is a versatile graphics manipulation package. This page should help you get a taste of what GIMP is capable of. You can also have a look at gimp introduction page.
Numerous digital photo imperfections can be easily compensated for using GIMP. Fix perspective distortion caused by lens tilt simply choosing the corrective mode in the transform tools. Eliminate lens' barrel distortion and vignetting with a powerful filter but a simple interface.
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.