Hey, awesome thread here! I'd also like to suggest Gimp, as Kamikazee said, along with Scribus and Inkscape. I believe these are all multi platform, though originally developed for Linux I think. Gimp is the freeware equivelant of Photoshop, and it ROCKS!!!! Scribus and Inkscape are the freeware equivalents to Adobe InDesign and Adobe Illustrator. One is a layout program (like for books, magazines, pdf's, etc.), and the other is for Vector drawing.
Give them a shot if you haven't yet, especially Gimp which works fantastic on a Microsoft Windows PC platform and you can find a lot of free brushes for it too, as well as plugins and such! I have used it myself and love it!
