Freemason's Hall, London

In the heart of London stands a huge, not entirely inconspicuous, grey building. This is the headquarters of the United Grand Lodge of England, the governing body of English and Welsh Freemasonry, and, surprisingly, its open to the public. Built in 1931, after the original 1776 building was declared structurally unsound and demolished, it was paid for by private contributions and stands as a memorial to those who gave their lives in the First World War. Hourly tours of the Halls nineteen lodge rooms and Grand Temple are available and you can also visit the library and museum, if you so desire - and no, you don't need to know the secret password or handshake to get in. Admission: free.