Hermitage Museum

Wikipedia (April 2011):

"Founded in 1764 by Catherine the Great and open to the public since 1852."
"Its collections, of which only a small part is on permanent display... including the largest collection of paintings in the world."
"Out of six buildings of the main museum complex, four, namely the Winter Palace (the former main residence of the Russian tsars), Small Hermitage, Old Hermitage and New Hermitage, are partially open to the public. The other two are Hermitage Theatre and the Reserve House. Originally, the only building housing the collection was the Small Hermitage."