Krita

Krita is an image editing application which comes as part of the KOffice project, released under the GPL.

Great colour space support and 16/32bit per channel float support are two of it's major drawcards for post-production. It also has some natural media colour spaces which may be particularly interest for matte painters. These colour spaces emulate the mixing and colour response of paints.

Your distribution's package manager will most likely have Krita available, though if not available as a standalone package, try installing KOffice.