I have several HDDs crammed in the drawers of my desk that I don't really have a use for. I have taken HDDs apart before just to see what was inside and a dude named Viacheslav Slavinsky has taken an old HDD and made one of the coolest clocks ever from the remains.
The dude etched numbers into the platter of the HDD somehow and strobes LEDs behind the numbers to show the time. Behind the platter is a PCB with its diffused LEDs in each character position that can be strobed independent of each other.
The platter of the HDD still spins crazy fast and as the platter spins and the LEDs strobe, you get a persistent display that shows the time. This is all kinds of cool; my kids have a couple foam fans that have words on the blades when they spin that uses similar tech. I wouldn't want to touch that platter while the clock is in use and it's rather loud to boot.