Several years ago I took a class called "Introduction to Modern Physics" in which I got my first taste of relativity and quantum mechanics. This was one of the most challenging and frustrating classes, both because the concepts were so radically different than the Newtonian brand of physics I was used to, and because the professor was just slightly above worthless.
In order to help wrap my lowly brain around the concept of spacetime, I began my usual routine of writing little interactive flash animations that displayed the equations and plots dynamically. Somehow this caught the attention of one of the other professors who was teaching an online course about Special Relativity, and he worked with me to create an interactive Spacetime diagram tool for use in his course. Here's what we came up with, along with some links for more information about Spacetime.