I use gimp as well. It does most of the stuff photoshop does. It isn't too hard to learn. There's also a version of it called GIMPshop that looks like photoshop (more or less).
But, I think the main problem with people wanting to edit their photos is that it can be hard to know what tools you need. Yeah, photoshop and gimp have everything you need if you're not shooting RAW, but you really only need a couple of tools that come with the programs. Here's a small list of things you should learn to use (can be found in both PS and Gimp):
Levels
-There is a histogram that tells you the spread of the colors by value. The other end is black, the other white. Usually you will want to adjust it so that there is somewhat even spread across the histogram. So, you drag the little arrows so that they are at the beginning and end of the curve. Or, you can do this with "Auto Levels"
-It's really useful to learn to read the histogram.
Hue/Saturation/Lightness
-If you take a pic so that the white balance isn't good (the pic is bluish or something), you can adjust it with the Hue. There are other tools for this as well, like Color Balance, which will give you more stuff you can adjust.
Crop
-Make a better composition by cropping your photos.
Layers
-Very handy to learn how to use layers, because then you can adjust different parts of the image to have different color tweaks. There are tutorials around. The basic idea is that you duplicate the photo and have it on the overlaying layer, then use different settings for it than the one under it. Then you can erase the parts of the overlaying photo to make the underlying photo come through in those parts. It's a bit too much to explain here.