Fourmilab Switzerland
John Walker: C Ship: The Dilation of Time
A good mathematical account of what happens to time as you approach c: the speed of light. Also provides a rather a fascinating video (fly through the Lattice) - don't miss it. Take some time and explore this site.
Georgia State University
Georgia State University: Hyper Physics: Michelson Morley Experiment
At this site from the Georgia State University is where the proof all starts. One must understand this experiment to have any confidence in relativity. Here's the experimental setup and a summary of the data.