I just started to use OpenCanvas. I stared with the free version 1.1. It was so fun to use that I bought the newest version which has quite a lot to it. It is definitely worth checking out. Not too expensive either. You can down load the original free version here, http://wistinga.online.fr/opencanvas/#downloads
For having a natural media look, nothing compares to Painter. That not being an option price wise, Painter Essentials is a low cost, light version of Painter, often bundled with Wacom products. A third program you might want to check out is Art Rage. It gives a simulated 3d bristle brush look and is also low cost. All of these programs have free trials.
Project Dogwaffle and Twisted Brush are two programmes that do most of what Painter does and in some cases more (Twisted Brush in particular has some mind blowing effects brushes). Painter is still the best but it is certainly worth checking these out as they're less expensive.