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Google Art Project http://www.googleartproject.com/
Explore museums from around the world, discover and view hundreds of artworks at incredible zoom levels, and even create and share your own collection of masterpieces.

TuxPaint http://ww.tuxpaint.org
Free paint program with extensive collection of stamps. Great for younger students.

ArtRage http://www.ambientdesign.com/artrage.html
Free natural media painting software. One of my favorites.

Sketch Studio http://www.sketchandsmudge.com/
Great paint software interface and it includes drawing tutorials. Uses basic chalk and smudge stick tools.

PD Particles http://www.thebest3d.com/pdp/index.html
Easily "paint" grass, trees, skies, and other landscape elements.

Sketch http://akvis.com/en/sketch/index.php
Converts photos into drawings and paintings.

Painter Essentials 4 http://www.corel.com/servlet/Satellite/us/en/Product/1190317151777
Full featured image editing and paint software. Great tools for converting photos into drawings and paintings.

UltraFractal http://www.ultrafractal.com/index.html
Software for creating fractal art.