Pixia is a great drawing and image editing software but totally free. It supports unlimited layers, 10240*10240 pixels layout, effect plug-ins(also accept Photoshop's plug-ins), undo 100 times. Also can working with your TWAIN devices(eg. Scanner or Digital Camera and Digitizer.
What's new in this version:
Version 6.01 adds a switch to turn OpenGL assisting function on/off on the View Control window.