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.
To install MyPaint open a Terminal window (Applications -> Accessories -> Terminal) and copy+paste the following lines:
sudo add-apt-repository ppa:tillux/t-misc
sudo apt-get update && sudo apt-get install mypaint
When installed you can find MyPaint under Applications -> Graphics -> MyPaint
MyPaint Homepage.
Some features are:
exists for several platforms
supports pressure sensitive graphics tablets
extensive brush creation and configuration options
unlimited canvas (you never have to resize)
basic layer support