I use photoshop CS5, but that shit's expensive (I got it for christmas a while back). A lot of people have success with gimp, which is free. I personally don't like it, but I haven't used it very much so I really just don't know my way around the program. Before I had photoshop, though, I used Paint.net, which I highly suggest. It's got a lot of the same stuff as photoshop, but a lot simpler and not as much stuff. It's also free.