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

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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.) 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, BSD and Solaris. It works well on slower machines (e.g., Pentium 133MHz), and on thin-client systems.