I'll list out a few.
#1 GIMP : Totally free, pretty damn good, steep learning curve when it comes to tools and the UI.
#2 Sketchbook Pro 2011 : Students get all Autodesk programs for free, though I don't know if this is international, and I don't know if you are a student.
#3 Inkscape : Again totally free. Focuses on vector graphics.
Those are the top ones I know about that are free.