I just discovered Krita and I think it's awesome.
Here is a video of a timelapse drawing done in Krita:
At this point I think Krita is the only Open Source design and drawing app that is close to PhotoShop quality wise.
It works on Windows and Linux as well as OsX.
You can download and install it free from here: https://krita.org/download/krita-desktop/
If you want to support the project you can do it here:
https://www.kickstarter.com/projects/kr ... e?ref=card