Light Speed! is a simulator which can illustrate the effects of
special relativity on the appearance of objects travelling at
ultra-high speeds. Depending on the particular speed, and one's
point of view, relativistic effects can cause the object to
appear shorter, longer, brighter, darker, deformed and/or
off-color.

To adjust the velocity, use the slider along the right edge of
the main window. You can also type in a value, using the entry
at the top right (press <Enter> once it is in). To change the
units shown, press the button to the right of the entry, and it
will cycle through a small list.

The object travels in the positive-x direction, which can be
visually checked by activating Objects -> Coordinate axes.