The beta version of Open Canvas has the distinct advantage of being free. Unfortunately it has the disadvantage of being mostly in Korean with a poor English translation.

Due to the simple fact that it is free, and that it has a heckuva lot of the features that Photoshop has, I'd say The Gimp would be a winner for the majority of people. I say that from the perspective of a digital painter, though, not someone who does photo touchups or graphic design, so their opinion may differ. I haven't used The Gimp extensively, but given the pricetag, you can't go wrong. And the fact that it is supported on Linux as well as Windows makes it attractive to many people.

ArtRage is in the same "family" as Sketchbook Pro and Painter in that it seeks to emulate actual real media. I can attest to its success at this...I was completely befuddled by how to make oil paints behave like I was used to. It was fun...5th-grade-art-class-fun.

I haven't used this too much yet, but here is a nice review that compares this to the "closest of type", ArtRage2 vs. Alias Sketchbook Pro.

Given the price, it sounds like it would be an excellent partner to Alias Sketchbook Pro, especially for tablet users. I hear the UI is exceptional.

(There is a version 4.5 out that I have not tried yet, so some of my old gripes may have been resolved for this program.)

You can't beat the price of Open Canvas, given that it's a fairly robust little program. It has a reputation as well for having fast brushes (which I can attest to), and my personal favorite of OC is that the zoom algoritm is superb. No matter if you are 100%, 38% or 50%, the image is always crisp (something I can't say for PS or Painter). It's also the only paint program I know of that lets you save videos of the drawing being made (ala Artpad). The downsides of OpenCanvas is that the support for non-asian customers is limited, and the text integration and manipulation tools are weak.

I have had very limited experience with Paint Shop Pro, so I can't give you a solid opinion. My general feeling about it is that, given the price, someone would be better served by Gimp or Open Canvas (unless you can get an academic version, in which case it is equal in price to OC), but it does seem that there are a lot of tutorials written for Paint Shop Pro. And since it is owned by Corel, you are likely to get better support in North America than you would for OC. Best I can say is...get the 30-day free trial and give it shot, then try Gimp, and decide whether it is better than a free program.

Sketchbook Pro is a tablet artists dream. To say it is intuitive would be an understatement. Unfortunately it really is intended to be used for sketching, and some basic things I like to do (like set layer styles to allow me to color linework easily or open existing .PSD files), it cannot do. Again, it is very good at what it does, but it has it's limitations. Not recommended if you don't have a digital tablet!

Ahhh, my first love. Painter is like the beatnic, rough-edged younger brother of Photoshop. It's intended first and foremost for painters, not for graphic designers or photo manipulation, and it shows. This amounts to some good and some bad. The good is that it has wonderful brushes and blending tools and paper textures and is just giddy with the creative fun-stuff. The bad news is that the color management is insanely confusing, the transformations are clunky, and it has a bad habit of crashing. Still, for pure power and capability, it's a close second to Photoshop, and it is cheaper (especially the academic version). If what you want is to digitally paint, this should be one of the first on your list. It isn't quite as good at mimicing real Paint as ArtRage is, but it is far more feature-full.

Arguably the most robust of them all, Photoshop is also the most expensive. While PS does not cater to the habits of digital painters like Corel Painter or Sketchbook Pro does, the array of transformation tools, layer styles, filters, tricks, and general gadgetry makes PS a program that it's hard to give up once you've gotten used to it. It handles many-layered files better than any other program. And because it is so popular, most tutorials on the internet will cater to PS users. Not to mention that nothing plays as well with the rest of the Adobe Suite (Illustrator, InDesign, etc) as well as Photoshop.