Rendera is a painting program for Linux and Windows. It's a little unusual in that brushstrokes are "rendered" in after being created by the user. This allows the use of more expensive painting effects and blending modes since they don't have to keep up in realtime.
Rendera doesn't have layers but allows editing the alpha channel, so masked images may be exported as PNG images (or TGA for compatibility with older software).
Windows beta release 0.2.5 (package contains both 32-bit and 64-bit versions):
Source code is available from GitHub.