TuxPaint

Tux Paint is a free drawing program designed for young children (kids ages 3 and up). It has a simple, easy-to-use interface, fun sound effects, and an encouraging cartoon mascot who helps guide children as they use the program. (See the full list of features. at http://www.newbreedsoftware.com/tuxpaint/features/) It provides a blank canvas and a variety of drawing tools to help your child be creative. Tux Paint runs on a variety of operating systems, including Windows, Mac OS X, Linux, FreeBSD and NetBSD. It even works well on slower machines (e.g., Pentium 133MHz) and on thin-client systems. Best of all, IT IS FREE.

http://www.newbreedsoftware.com/tuxpaint/