Paint.NET is one of the better free paint programs that is available for doing basic image and photo manipulation. Let me be clear, Paint.NET is not a Adobe Photoshop replacement. Although, if you have some basic image editing needs that Microsoft Paint doesn't meet, then check out this program.
Some of the Paint.NET's features include: layer support, unlimited undo, special effects, simple tools for drawing shapes, and more. For a more complete list of features, or to see some screen shots.