Getting Around in GIMP - Adjusting Levels
In my previous post on Color Correction, I outlined a method for finding the lightest and darkest values in an image using Colors → Threshold. What I may not have mentioned in that post was that the process described will actually level balance your images for you automatically if you follow the steps (choosing the darkest and lightest pixel values to represent black and white in your image).
If you were just interested in basic level adjustments, I thought I would cover that here.