Tux Paint is a free, award-winning drawing program for children ages 3 to 12. It combines an easy-to-use interface, fun sound effects, and an encouraging cartoon mascot who guides children as they use the program.

My kids just love this program, and how can you beat the cost since it's totally free (it's part of an open-source project).  We use the PC version of the software, but on the download page, it appears to be supported on:

• PCs
• Macs
• Linux

This is something you really need to check out because its one of the few things out there that is really free and does not even have any advertising in it!