David Briggs' The Dimensions of Colour, a comprehensive online explanation of traditional (what you've probably been taught) and modern colour theory, and its applications to visual art. Invaluable for artists and non-artists alike. (The answer: probably some kind of brown. Yes, your kindergarten teachers fed you lies.)
Japanese scientists think they may have an explanation for how a three-dimensional universe emerged from the original nine dimensions (plus time) of space: the universe had nine spatial dimensions at its birth, but only three of them experienced expansion. A Hollywood sound designer tries to explain ten dimensions. Efforts to portray an eighth dimension have already been explored.
Something has happened to the world. Everything has been flattened into 2D - except you. Now the world needs you to put things back to normal. Antimatiere is a small puzzle game where you move 2D objects around on walls and floors to solve problems. It also has a slight resemblence to Portal in that placing doors and windows punches holes through the mentioned walls, allowing you to pass between different rooms. Warning: Requires the Unity Web Player plugin to run. [more inside]
The London Underground. Every Londoner has used it, but has everyone really seen it? The old map is looking a bit dusty. Perhaps its time for Geographic precision or maybe 3D projection. If we add bicycles to the map, is it still an underground? [more inside]
Mathematicians create videos that help in visualizing four-dimensional objects. Science News writes about it: seeing in four dimensions.
Anybody remember Slow Bob In The Lower Dimensions? Turns out the short video, once a mainstay of early 90s late-night MTV, was created by one Henry Selick, director of, oh, The Nightmare Before Christmas, James and the Giant Peach, Monkeybone, and the forthcoming adaptation of Neil Gaiman's Coraline. A lot more on Selick; also, higher quality, alternate format (but slower loading) versions are available here.
Sizeasy. A handy visual tool for comparing all manner of different rectangular-shaped things.
Imagining the Tenth Dimension (Flash). 10th dimensional physics and string theory don't get any easier than this.
A small world in most dimensions: "University of Washington scientists using gravity measurements to hunt for evidence of dimensions in addition to those already known have found that those dimensions would have to occupy a space smaller than 0.2 millimeter."