I was watching a timelapse video of a guy painting with My Paint that I posted around here somewhere and decided to give it a try. I love how it lets you paint without too much interaction with the actual application letting the artist consitrate on his work. It works with Intuos4 and it has a great color picker that shows complimatary colors which is a must for an artist.
And of coarse it is free and open source. It can be found here:
http://mypaint.intilinux.com/?page_id=6