Guess who? I'd have guessed the FBI. Or the Library of Congress. Or a private collector with an interest in the area – someone like Larry Flynt. But nope, the biggest collection of banned books is owned by the Catholic Church. Their List of Forbidden Books is an index of books that the Church has considered against the faith since 1529. The index was created as a guide to a semi-secret library that is housed at the Vatican and which helped the Vatican control the flow of information for over 400 years.
[2007-03-11 – correction. According to Snopes which is an authority on hoaxes of all types, this story is not true. But then again, they only cite the Vatican librarian as the source for this statement and you might not expect this person to be up front about something like this. Truth or urban legend? Either way, it makes a good story.]