Science Friday (that's an allusion to National Public Radio's Talk of the Nation, by the way): "Lagrangian points", or "Trojan points", are orbital positions which balance the gravity of two different bodies. These equilibrium points make excellent locations for space stations.Building a real space city in the Lagrange point which trails behind Earth's moon would be an awesome -- yet feasible -- accomplishment (via the Babylon 5 History Page/Planets of the Earth Alliance). The L-5 Society is dedicated to producing such a station. Reading on we find that, of course, it was Arthur C Clarke's idea first. I ended up posting this article over at MeFi and there's a little discussion going on.