I just discovered this thread, even after all the time I spent on this forum. I love museums, and I'm really glad to see a thread like this, and thanks to geomorph, there seems to be plenty of activity. Now, I assume that most of the major museums in Philly, New York, DC, and Chicago I've been to are already here, so here's a smaller one that I really love.
Museum: Brandywine River Museum
Location: Chadds Ford, Pennsylvania