Book Mooch is described as :-
BookMooch is a community for exchanging used books.
BookMooch lets you give away books you no longer need in exchange for books you really want.
Although I discovered it today on the excellent Libvibes, and its already has large coverage on Technorati.
Its a great idea. Problem is like many people I hate getting rid of books (unless there from relatives and never what I want). Anyhow, looks good. Now let's see how many copy the idea.