C.L. Hanson at Letters From A Broad posted an essay about the public/private divide in which she uses the analogy of a lighthouse.
I think the classic example from economics 101 is the lighthouse. In a costal city, no private individual or corporation profits enough from the lighthouse to pay the entire cost of building it, but the entire city profits from the trade that is made possible by affording ships safe access to the local harbor.
Well, either I didn't get that analogy, or I didn't remember it from Economics, so I'll try to remember it for the next time it comes up. (Emily really hopes it doesn't come up.)