Update: 7/20/2016 - I haven't maintained this list in years, primarily because I no longer have access to a computer. I simply can't try programs out anymore. If you want to expand the list, feel free to help by providing reviews, screenshots and links to your program of choice. In the meantime, if people are interested in reviews of iPad/iOS art apps, I can provide that. Otherwise I will not respond to art program suggestions. Sorry.
Photoshop is a great program, but despite it's many features it's a bit pricy at $999. I created this list for people with smaller wallets and certain art needs.
The programs are listed according to what type