M.L.
picnik and photoscape are free, but to do some things you have to upgrade which costs about $25 a year for picnic. I use it and love it.
here are some picnik tutorials I have written:
faces tools:
http://littlelovables.blogspot.com/2009/12/i-heart-faces....
color editing:
http://littlelovables.blogspot.com/2010/02/photo-editing-...
special effects:
http://littlelovables.blogspot.com/2009/07/editing-your-p...
simple heart bokeh:
http://littlelovables.blogspot.com/2010/03/creating-heart...
photoscape tutorial:
http://littlelovables.blogspot.com/2009/07/fancying-photo...
picasa is one of the best for just simple touchups, and it's free, downloadable (get it from cnet.com) and it uploads pics automatically so you don't have to do it one at a time, just work on the photo and export it to a photo folder.
adobe photoshop is the best, though a little pricey, but will pay off in the end, especially if you want the photos to be printed, and there are a lot of tutorials. gimp is like photoshop, free a little more spotty, but also still a lot of tutorials out there, though not as many as photoshop. I believe most photoshop brushes and stamps will work with gimp.