Artportal Fun + Games
Drawing Tools
ArtPad - Very simple online paint tool.
QUEEKY - Slightly more advanced drawing tool. Has a "Multidraw" feature, which appears to be the ability for 2 users to collaborate on one picture.
Saatchi Gallery Painting Studio - Fun and not a bad tool, but still far less sophisticated than true graphics software.
Picnik - Online photo editing tool. Crop, resize, sharpen, etc.

Artist-Inspired Drawing Tools
Picasso: Mr. Picassohead
Mondrian: Mondrimat
Mondrian: Make Your Own Mondrian
Mondrian: PAC-Mondrian (actually a PAC-Man type game)
Jackson Pollock: "Jackson Pollock" online drawing tool (I'm not sure how faithful it is to Pollock's art, but I defy you to make a non-abstract painting using it.)
David Hockney: "Hockneyizer" - upload a photograph and it creates a mock Hockney-style collage.

Links to Activities Art Games Recommendations
Incredible Art Department Activities & Games List

Museum Activity Pages
Albright-Knox Art Gallery ArtGames
ArtisanCam - Scads of interesting games
Birmingham Museum and Art Gallery, UK - For Kids
Cleveland Museum of Art - CMA Kids
Metropolitan Museum of Art - Just for Fun
National Gallery of Art - NGAKids Page
National Museum of Wildlife Art Games Page
Seattle Art Museum - Mini-sites
Smithsonian Education - Games & Activities
Smithsonian Kids
Victoria Art Gallery, Bath, UK - Match the sound effect to the painting
Yale University Art Gallery Games
  Paper Crafts
Printable paper bird sculptures from illustrator Josh McKible.
Paper Robots
RavensBlight - Printable "Haunted" Paper Toys, from a medieval helmet to a battle-hearse.
Origami art blog with a variety of free projects described
"First Person Shooter" glasses. From artist Aram Bartholl.

Optical Illusions
Printable optical illusion dragon sculpture. From Grand Illusions Toy Shop.
Akiyoshi's Illusion Pages. Tons of examples, mainly of "anomalous motion" (static images that appear to rotate or vibrate).
Mighty Optical Illusions website. This particular page has many examples of trompe l'oeil ("fool the eye") graffiti.
Article on the illusionistic photographer Olivo Barbieri. He uses a special tool called a tilt-shift lens to... well, you have to see it for yourself.

Visual Math
JW's Pictures and Patterns Site - A variety of interactive tools that allow you to experiment with fractals, cellular automata, Spirograph patterns, etc.
Mathematica Demonstrations - See and interact with a wide array of mathematical visualizations

Very Miscellaneous
Strange Maps Blog - Bottomless supply of offbeat maps being used to convey all sorts of conceptual information. The examples are sometimes quaint, sometimes brilliant, but always thought-provoking.
Website for LEGO Sculptor Nathan Sawaya
Brickshelf - gallery of LEGO sculptures
Art Historians' Guide to the Movies - What was the name of that painting in that movie?
NOTCOT- Playful Design
Visit a random Wikipedia page every time you click this link
Visit a random link from the Yahoo directory