Drew Olbrich's homepage has a number of interesting things. This page clears up misconceptions about the distance between the earth and the moon relative to their sizes (it's probably a lot farther than you thought). This page helps you intuitively understand why a rotating 4D hypercube projected onto 3-space (then probably onto 2-space) looks the way it does. He made this ultra-simple 3D input device out of nothing but two joysticks, a ball, and some rubber bands.