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.
Physics Classroom
The Physics Classroom: Motion and Forces in Two Dimensions: Relative Velocity
Through illustrated examples and interactive practice problems, students explore problems that involve relative velocity- where the actual motion of an object is different than its motion relative to a stationary observer.