Krita

Krita is a bitmap editing program that supports ICC-profiles, the CMYK color model and 16-bit colors.

From the Krita site:

Krita is a creative sketching and painting application based on Calligra technology. Whether you want to create art paintings, cartoons, concept art or textures, Krita supports most graphics tablets out of the box. Krita’s vision statement is:

  • Krita is a KDE program for sketching and painting, offering an end–to–end solution for creating digital painting files from scratch by masters.
  • Fields of painting that Krita explicitly supports are concept art, creation of comics and textures for rendering.
  • Modelled on existing real-world painting materials and workflows, Krita supports creative working by getting out of the way and with snappy response.

Comments

Krita is not essentially a bitmap editing software but instead a "illustration from scratch" software. The difference between it and MyPaint is that MyPaints goal is to be a natural drawing environment where as Krita strives more for an open environment where natural brushes and canvas effects are available.
It shares allot of effects with Alchemy so that it can be used as a sketching program.

One of the best details available is its assistance software for illustrators who can't afford or do not own a wacom tablet but instead use an ordinary mouse.

It has also just come out in a touch-version called Krita Sketch, for the moment only available on a few tablets.

There is a large amount of tutorials for Krita made by users - as for example White Herons collection on DeviantArt:
http://white-heron.deviantart.com/art/Krita-tutorial-Basics-325962811