The funniest demonstration that I watched was at the computer museum at the University of Stuttgart (it's just a single room, but it contains a lot of history!). The guide took an old, butchered radio that was reduced to a coil attached to a speaker and put on top of the front panel of a PDP-8. Then he started a Fortran compiler, which would take several seconds to complete. During that time, the radio made kind if hideous digital beeping noises from the CPU's EMV radiation that got picked up by the coil inside. You could easily learn to distinguish different compiler phases and tell whether the program made progress. The guide explained that this was a common way for operators back in the day to keep track of the jobs they were running while taking care of other tasks: were they still running? Did they get stuck? Did the job complete and is it time for the next one? Some inventive guys figured out that when you wrote certain instruction sequences, the EMV noise would become tonal and the pitch could even be tuned to some extent. That got them to write programs which would compute nonsense, but when you picked up the EMV emissions, you would hear music! The museum guide ran a few of these programs to our great amusement :).

I've yet to see this mentioned - or demonstrated - anywhere else.