[adrianmonk]

One of the most delightful and hilarious things I've ever seen anyone do with a computer involved a C64 and its parallel port. This guy I sorta knew in college had built a box with a bunch of relays and 120V AC power outlets. The parallel port would control the relays. Into this box, he plugged a bunch of lamps, then he'd turn out the other lights and play music on the computer, and the lamp would basically create a light show in time with the music.

He also had written some kind of memory-resident program (a TSR in DOS terms) that he could trigger while another program was running, and it would render pages of memory to the screen in real time. So he could start any program that had music, browse around through its memory space until he found a byte that seemed to be updating in time to the music, and then select that as the byte whose value got periodically copied to the parallel port.

So you could load up your favorite video game and then have a light show in sync with its music. Or any other activity of interest.