So I've been obsessed with this ArtRage software for the last 24 hours or so.
There is a free starter edition at the site. It has all the basic tools you need and does a damn good job at emulating placing paint on a surface. Full version is a whopping $25 and includes extras like importing stencils, extra modes for the painting tools, metallic paints, and dumping paint straight from the tube (you can even make your own little mixing tray and do it the old-fashioned way).
Here's what I've been up to with it in the last 24 hours anyway.