Bouncing a focused beam of radio waves off the moon, and listening for the echo. Pretty cool, you can send data, audio and still photos that way.
The first to do it were the Apollo astronauts. Since then, it’s been a project that many HAM radio enthusiasts enjoy, because it involves some nicely sophisticated targeting, a big dish, noise-reduction technology…
Last year there was a rather large citizens’ Moon Bounce festival, drawing kids into it from all over the world to learn about the space program, about physics and radio technology.

A radio telescope operated by the Stanford Research Institute was one of may which participated in the first Echoes of Apollo event.
One of the founders of Prevocative, Michael North, is also a founder and adviser to Echoes of Apollo. In fact, their wonderful website is hosted on the same server as ours:
One of the leading HAM journals, CQ, tells the whole story in an entertainingly geeky way…the website tells about upcoming events, one of which will involve the world’s largest radio telescope at Mt. Arecibo.