MyPaint is a fast and easy open-source graphics application for digital painters. It lets you focus on the art instead of the program. You work on your canvas with minimum distractions, bringing up the interface only when you need it.
Available for Windows and GNU/Linux
Designed for pressure sensitive graphics tablets
Simple and minimalistic user interface
Extensive brush creation and configuration options
Unlimited canvas (you never have to resize)
Basic layer support
This software is simply awesome. Totally recommended!