This open-source program has long been a staple of the Linux world, but a Windows version has been available for almost as long. Its user interface isn't as intuitive as I would like but it is more powerful than Paint.NET. GIMP is unique in the world of graphic manipulation programs because it doesn't really have a main window with everything else inside. When you start GIMP, you get two windows full of tools. Each image opens in its own window, which makes management a nightmare when you have more than a few images open. Both the tools window and the image window have menus, making the interface even more confusing.
GIMP has several options for automatic photo processing, adjusting the white balance, contrast, brightness, and HSV (but no red-eye removal that I could find). However, the automatic results never looked that good, leaving me to attempt to adjust the images by hand. The adjustment dialogs show the effects of any adjustment before applying it to the image, but I still wasn't able to fix images to my satisfaction.
On the positive side, GIMP is very powerful, and like Paint.NET it has a plethora of plug-ins available (some of which might offer automatic enhancement). It also has a thriving user community and excellent documentation. On the negative side, it just isn't easy to use. I suspect that it may be configurable to eliminate several of my issues but I was unable to do so. Sadly, the GIMP crashed to a blue screen several times on two different computers.