The Eyeballing Game

There's a really nifty game online called "The Eyeballing Game." The game is just what it sounds like; you "eyeball" distances, angles, and locations based on existing lines, locations, or circles. It's aimed at woodworkers, but I think that it achieves an unintended success of being a good mental calibration tool for artists. Accuracy in drawing is all about being able to judge spacial relationships. With this game, it's a bit like exercise for that part of the brain.

http://woodgears.ca/eyeball