Good drawing utilities

Here are two very good drawing utilities

Gimp:
For mac: http://www.gimp.org/macintosh/
For windows: http://www.gimp.org/windows/

Inkscape:
Both mac and windows: http://inkscape.org/downloads/

Here is a good website for Inkscape tutorials: http://2dgameartforprogrammers.blogspot.com.au/