I use paint.net. It's free, and I think it's a really great program, it's really simple to use if you've tried photoshop before. GIMP, at least in my opinion, would require more in regards to learning, especially if you're used to photoshop. The only downside to paint.net is that it lacks a smudge/blur tool, (for now, I've read that they plan to include it in later releases) which I heard the GIMP has (correct me if I'm wrong...). You can also create all sorts of plug-ins for Paint.net, and you can download them from their site.
Anyways, I want to know how many people use graphics tablets, and which one? I'm a poor college student who is looking for a graphics tablet for under $200, and I could really use some help and suggestions. :)