Everything in the known universe, created by 14-year-old twins.
After you follow the link, click "Start," and then use the slider across the bottom, or the wheel on your mouse, to zoom in -- and in and in and in... or out and out and out... It will take you from the very smallest features postulated by scientists (the strings in string theory) to the very largest (the observable universe). This really is a fabulous visual demonstration of scale at micro and macro levels. This is an excellent way to bring spatial thinking into the math curriculum as well. See this on the twins website at: http://htwins.net/scale2/
Via Seth Dixon, FCHSAPGEO