Just to add a bit of context to those replies:
GIMP is a superb program, which is comparable to Photoshop. However that means that (just like Photoshop) it involves quite a steep learning curve to get the best from it. If you're looking to emulate the the standards of professional art editors, that's what you need. Otherwise you should go for something simpler.
(If you do choose to download GIMP, I strongly advise downloaded the full documentation as well).
Paint.net is also fairly complex to use (but probably easier to initially get to grips with).
For simpler photo processing, Google's Picasa software is highly recommended:
http://picasa.google.co.uk/
Serif PhotoPlus SE is a cut-down version of a full commercial program. It's easy to use and well worth a look:
http://www.serif.com/...ware/?MC=FSSPHOTOPLUS
Chris