I have not used Gimp, but I know it's the open-source free version of Photoshop. A lot of people like it.
I use PhotoFiltre, which is okay. I don't love it, but it's okay. It was free, though, and it works for basic photo manipulation (tweaking contrast/cropping/resizing/etc). When I get a new computer, though, I think I'm going to try Gimp.