Here's a list of free ones. ^^ There are probably a multitude more out there.

openCanvas 1.1 http://wistinga.online.fr/opencanvas/
GIMP http://www.gimp.org/
Artrage http://www.ambientdesign.com/artrage.html
Pixia http://park18.wakwak.com/~pixia/
Artweaver http://www.artweaver.de/index.php?en_version