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.
Massachusetts Institute of Technology
Mit: Open Course Ware: Courses: Physics: Introduction to Special Relativity
College-level electrical physics course highlighting Einstein's Special Theory of Relativity. Course introduces the physics of Lorentz contraction, time dilation, the "twin paradox", and E=mc2. Course features suggested readings,...
University of Virginia
University Virginia/time Dilation: A Worked Example
Here's a solved problem in time dilation. It's still not easy to follow, but it's doable. The math is not difficult, but the mathematical thinking is demanding.
Other
Walter Fendt: Ejemplo De Dilatacion Del Tiempo
This simulation shows that the ship clock goes slower than the two clocks of the system Earth and Pluto. This site is best viewed with Internet Explorer.