The Benjamin Franklin House is the only one in the world. He lived here from 1757-1775. He lived here for 16 years and he was here working on the eve of the American Revolution. It was also used as the first US Embassy.
It is an interactive museum so they take you around the house and have a blend of actors and technology which puts an interesting twist on the presentation.
It is near Charing Cross so if you are doing anything out that way, its definitely a place where you should stop by. If you travel by train you can get in using the 2 for 1 coupon by Days Out Guide.