Ramanujan and Hardy are two well-known mathematicians who collaborated in Britain between 1914 and 1920. One day Hardy took taxi number 1729 to visit Ramanujan, and remarked to him that he (Hardy) could think of nothing special about the number (1729). Ramanujan replied that it was in fact the first number expressible as the sum of two cubes in two different ways (1^3 + 12^3 = 9^3 + 10^3). The story has since become an anecdotal symbol of Ramanujan's brilliant mind.

http://en.wikipedia.org/wiki/1729_(number)

That's a very obscure reference, though, so I apologize for sending anyone looking based on my previous comment.