A few linksI found about two dozen digital photography tutorial links on the net while putting this website together. The short list below seemed to be the most interesting. There is a lot of good information out there.
Installing GimpGo to http://gimp.org/downloads and click the download link suitable for your operating system. Gimp can be installed on Windows, Mac or Linux. I use linux most of the time. But for Gimp it doesn't matter. Gimp looks and runs the same on all platforms.
Gimp Levels Documentation
Video TutorialsGimp Introduction
Color Correcting with Levels
More Hue Saturation