Factory/Plant Tours - Where Would You Go?

kingelvis asks: "I have been thinking of putting together a road trip with a theme - Traveling to factories, manufacturing plants, etc all over the country and taking their (free) tours. I've already thought of a few places; Boeing, Auto factories in Detriot, Ben & Jerry's, Jack Daniel's distillery, and so forth. I'm interested in everything from 747s to bottled water, so please respond with any public tour you can think of. Where are some cool places you'd recommend visiting to see how stuff gets made? "