What LEGO pieces have "real-world" functionality?
All Aboard!

I'd like to add my all-time favorite LEGO piece:


x870cc02, THE BLACK WHISTLE!




The white brick is a microphone. Attach it to your train's motor and BLOW THAT WHISTLE!  Depending on how short (or LONG!) your whistling was, the train would go forwards or backwards. 


Real-world use: Not only did it drive your train round the bend, but your parents too...!