What is London's best undiscovered museum?

Aladdin's cave!

Apparently they do candlelit tours once a month, which could be pretty romantic.

http://www.soane.org/  

Not a bad visit, the Horniman. I went on a Saturday afternoon when the place was overrun with children and toddlers all having a very jolly time. Must come highly recommended by a mother's group somewhere. It's mainly natural history stuff and the exhibits often seem to have been randomly allocated space in cabinets. There's also a half-decent aquarium in the basement! And if you get bored inside, take a wander outside and peer out over south London from the gardens. Mine was certainly an afternoon well spent.  

surprisingly interesting  

Britain's original home for abandoned children and London's first ever public art gallery


The Foundling Museum

40 Brunswick Square

London WC1N 1AZ
020 7841 3600 // enquiries@foundlingmuseum.org.uk

Tues–Sat 10am–5pm, Sun 11am – 5pm

CLOSED ON MONDAYS

Admission £7.50 // concessions £5 // children free
The Foundling Museum tells the story of the Foundling Hospital, London's first home for abandoned children and of three major figures in British history: its campaigning founder the philanthropist Thomas Coram, the artist William Hogarth and the composer George Frideric Handel. This remarkable collection of art, period interiors and social history is now housed in a restored and refurbished building adjacent to the original site of the Hospital, demolished in 1928.
 

Not strictly a museum, but a fantastic experience, as you explore a Spitalfields Hugenot house through time. An absolute must-see, though be warned, the opening hours are limited. More here
http://www.dennissevershouse.co.uk/  

I went to the Museum of Brands & Advertising recently in Notting Hill. It's small, but absolutely fantastic!  

Interesting Museum  

Musuem of Surgery - strange specimens and instruments from the Victorian era up to today  

Try the Geffrye Museum. House interiors from 1600 to now in a lovely setting. Close to Columbia Road, Shoreditch etc so you can include a visit in your Sunday bagel/flowers run.
http://www.geffrye-museum.org.uk/