You're using it every time you buy airline tickets. The vast majority of Dynamic Pricing algorithms are just PID algorithms that are designed to maintain sales rates such that the last ticket is sold at flight time.
Last Fall my roommate was taking a class on computer networks, he ended up inventing PID as a flow-control algorithm, though he didn't know to call it PID.
Oddly enough I too unknowingly implemented a PID controller without prior exposure to the theory. I was trying to make a stable hovering platform using only thrusters in Garry's Mod. I remember once I tweaked the parameters just right it was amazingly responsive and stable. I only realized what I had actually done years later.