University of New South Wales (Australia)
University of New South Wales: Einstein Light
Einstein Light highlights the Theory of Special Relativity and related topics. Learn how Galileo, Maxwell, and Einstein contributed to our knowledge of relativity, electricity, magnetism, and time by watching fun, interactive modules.
American Association of Physics Teachers
Com Padre Digital Library: Open Source Physics: Simultaneity Spacetime Diagram
Manipulate variables in order to observe the effects of relative motion on events such as an explosion and the time at which its light signal reaches a destination.
Other
No Carolina St Univ: The Day the Universe Went All Funny
This site provides a very readable treatment of a really difficult subject. You still have to work hard at it, but this article can make relativity understandable.