Tux Paint is a simple and entertaining drawing program geared towards young children. It has a simple interface, sound effects, and a cartoon character (Tux, the Linux penguin). Along with drawing brush strokes, lines and shapes, you can also enter text and place "rubber stamp" (or "sticker") images on the picture. Tux Paint is extensible, and could be useful in an educational environment (such as a grammar, elementary, or grade school). It's portable across numerous platforms, and runs well even on slower systems like the Pentium 133MHz.
Pixel is an application for editing, retouching, manipulating, and animating RGB, CMYK, and HDR images. It was formerly known as Pixel32. It is available for Windows, Linux, Linspire, Mac OS X, BeOS, Zeta, QNX, MorphOS, FreeBSD, eComStation, OS/2, SkyOS and even old plain DOS, for both x86 and PowerPC architectures.