Krita
Krita is a graphics application for everyone who wants to get creative with images. Whether you want to create from scratch or work with existing images, Krita is meant for you.
Krita is enormously flexible and allows you to work the way you want, but by default presents you an uncluttered interface where everything is where you expect it.
There are both fun and easy-to-use features like guided painting, and advanced features like 16 bit — and higher! colorspaces, layer groups or tonemapping.
Krita is free software and is therefore under continuous development. We welcome contributions, whether that is code, documentation, tutorials, art for our gallery or ideas and expertise from artists and users. Join us on #koffice at irc.freenode.net, or on our mailing list. Ideas are developed in our wiki. Krita can only grow through your contributions!