The Gimp

Paint.NET

By far, the most sophisticated free image editor ever. It also has the most active development community which means it will keep getting better and better. The only people that need something that it doesn't offer are the pros that have to use Photoshop. Some (heck, many) have complained that the interface is non-intuitive, and there is some validity to the complaint, but the loudest complainers are that same Photoshop crowd who resist it just because it doesn't look exactly like Photoshop. For serious image editing that's totally free, the Gimp is your only real choice.

Now, having said all those complimentary things about the Gimp, I feel obligated to explain why I included Paint.NET. First, it is not as powerful as the Gimp, but it is probably more powerful than most of us need. Second, it has a more traditional interface and is easier to learn. Third, it is very well designed and is significantly smaller (about 5 meg). It's a MS Paint replacement, not a Photoshop killer. Paint.NET was developed and is maintained by Washington State University. Nice job, guys.

Either of these tools are excellent, free photo editing software choices.

http://www.gimp.org/

http://www.getpaint.net/