MyPaint! - http://mypaint.intilinux.com/
I recently found this one and had just started using it. I'm not sure if anyone would be interested but it is open-source and looks and feels very well done.
Krita (http://www.kogmbh.com/download.html) just came out for Windows as well, but it seems pretty far from being stable. It is in experemental stage at the momemnt. MyPaint on the other hand seems very stable.
I'll try to have a development test up in the next day or so.
Addendum: I'm still going to post a beta of 1.0.0, but it won't be submitted for official status any time soon due to issues with the latest version (1.1.0) being practically unusable on Windows.
If they get the issue fixed then I will think about submitting it.
Thanks for considering. No rush though, I would love to see it fully functional as well before going final with it. Many thanks again.
its out.