If you like Google Sketchup, it seems to now be available for the Mac. This is probably old news but I really like the program (although I should confess I don’t use it a ton). If you have not tried it you might want to since it is free. It is a 3D software tool. It is interesting from a UI perspective in that it has a limited number of tools yet is pretty powerful.
Another free and cool program is paint.net. It is for Windows only. This is more of an image manipulation program.