My digital drawing & painting program of choice these days is the open source software, Krita. It’s got great brush engines, works fantastically with my Wacom tablet under Linux (my primary criteria), and they also recently launched a well supported Windows version.
They recently started a Kickstarter campaign to fund some full-time developers who’ve already made significant contributions to the project.
If you haven’t seen it before, check out: http://www.krita.org/
If you’re interested in supporting its development: https://www.kickstarter.com/projects/krita/krita-open-source-digital-painting-accelerate-deve/