I see several folks bashing GIMP...don't really understand why. I use it for my pen photography and I think it's great. Like the other products mentioned it has tons of features, and also just like the other products you really only need a tiny fraction of those features for pen photography. And by the way, the features that i use are the same features you would use in the other products. Here they are:
Levels - use the adjust the color balance against a white card I include in each shot.
Curves - used to adjust the overall brightness to my liking (works much better than brightness-contrast feature)
Crop Tool - use the frame just the portion of the photo I want. It also allows me to eliminate the white card from the image.
Resize Tool - allows me to resize the image based on what I'm going to be using it for.
Unsharp Mask - when used at 1/2 the default settings, it magically sharpens the image.
I occasionally use the Airbrush Tool to clean up dust spots, but I find I can clean the pen and retake the shot much faster, so I pretty much have stopped using it.
I do other things with GIMP that require the use of other tools, but these 5-6 are all I need for pen photography. There's nothing unique about them, they're the same tools, used for the same purpose as in most other photo editing software. Once I load the photo I want to edit, using the five tools mentioned above, I can have a web ready image completed and saved in less than a minute. I'm not sure why some folks are having trouble using GIMP for pen photography. I'm no graphics expert and was able to learn to do what works for me in an afternoon. I think its a given that you have to start with a somewhat decent photograph. No software is going to fix an out of focus shot, or one that's severely over or unner exposed, or a bad composition.