This is pretty useful too -- especially with ellipsoids (3-D ellipses).

Here's an ellipsoidal reflector spotlight.

You put the light bulb at one focus...  and an actor at the other!

And what about reflecting sound?

Some science museums have what's called "whisper rooms." The ceiling is an ellipsoid...

If you stand at one focus... and whisper... someone standing at the other focus will be able to hear you over a noisy crowd!