Dec 2007:
Many of the links below are now broken,
including several whose passing is definitely regrettable.
The need for lists like this has diminished
thanks to Google's remarkable search algorithm.
Given the web's rapid pace of change,
a list can at best be ephemeral unless one spends a lot of time
keeping it up to date.
I've left this page up for history's sake.
John Walker's
C-ship,
with images and animations of what it looks like to travel at relativistic
speeds
(nice idea, beautiful graphics, but the images aren't correct yet -
you need to fix this, John Walker)
and
Interactive Experiments in Gravity,
illustrating various aspects of general relativity.