I’m from New York City, which means that any space we can conserve is precious.  That is why, getting a trundle bed as a kid was just about the best thing ever–finally, I could have friends sleep over in my room (and on a bed), without actually having to have 2 beds in my room.  While perhaps not the most glamorous invention, the functional benefit of having a trundle bed has been enormous for the many years I used it throughout lower, middle and high school.