Oh this bit made me so happy too! My high school science fair project involved controlling stepper motors off a parallel port. I really didn't want to fry the port -- I needed it to print out papers.
So I used a collection of optocouplers to glue it all together. Good times.
When Thomas Massie was a child, he was fortunate enough that his parents bought a computer, and he desperately wanted to start making robots with it. However, he was smart enough to know that it was a bad idea to start sticking wires into the family computer that cost thousands of dollars. So, Thomas hatched a plan. He figured out that he could scotch-tape photo sensors to the computer screen and write programs that turned portions of the screen either on full brightness or full darkness. That way, he could write programs that controlled motors, without electrically connecting anything to the computer itself.