St. Stephen's Tower
The 318 feet high St. Stephen's Tower (the Westminster Clock Tower), popularly known as "Big Ben" (this is a popular misconception) , is part of the Houses of Parliament, and situated at the north end next to [Westminster Bridge]?.
Big Ben is the name of one of the bells, and was supposedly named either after Sir Benjamin Hall, the Commissioner of Works at the time (and a large man), or Ben Caunt, a popular boxer in 1858, when the bell was cast. Big Ben the bell itself weighs no less than thirteen tons, and once could be heard in calm weather over the greater part of London.
Although not generally open to the public, you can arrange a tour through your MP.