Perhaps you're familiar with Corel's natural media painting application Painter. If you've ever had the desire to play around with it, but can't justify the expense, check out Ambient Design's free Artrage. It's not nearly as feature-full as Corel's product, but, like any product hitting version 9, Painter is showing signs of bloat, whereas Artrage is simple, svelte, and focused on its core function. In terms of the final results, you can see from the samples on Ambient Design's website that Artrage is a capable little tool. It's available for both Mac and Windows and works best if you have a tablet or other pressure sensitive input device, though it does not require one.