Google Art Project
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.

Free paint program with extensive collection of stamps. Great for younger students.

Free natural media painting software. One of my favorites.

Sketch Studio
Great paint software interface and it includes drawing tutorials. Uses basic chalk and smudge stick tools.

PD Particles
Easily "paint" grass, trees, skies, and other landscape elements.

Converts photos into drawings and paintings.

Painter Essentials 4
Full featured image editing and paint software. Great tools for converting photos into drawings and paintings.

Software for creating fractal art.