So, according to Wikipedia, that unimpeachable source, the only subway systems in the world that operate 24 hours, 7 days a week are the New York City Subway, PATH, the Red and Blue lines of the Chicago L, and PATCO between Philadelphia and its New Jersey suburbs.
Anyone know of any others? Night bus networks, while nice, don't count.
(I thought of this after a conversation with bloodstones about Chicago's Blue Line, in which she said, "All-night transit service is a mark of civilization. Sorry, Boston.")