Was at the Seattle Art Museum from 2-4 am, for part of the "35 hour marathon" for the Grand Reopening. The whole museum was free, provided you had a ticket, and from 1am onwards there was a DJ and a dancefloor, there were drinks and food during legal serving hours, and the gift shops were open. A pretty good move on the Museum's part -- I've never seen that many people under 40 in an art museum before.
It was kind of funny, you could hear the techno music everywhere in the museum, which gave the modern art section an appropriately Euro feel; but it didn't quite mesh with the pre-20th century exhibits, the African masks, or a bunch of other wings. But the whole affair was really cool. I like it when people are out and about in a city.