I thought of starting this topic when I read Kevin's overrated one. There have been times in all of our travels where we decided to just check something out only to be pleasantly surprised by what we got. What was one of those times, especially with museums or other sights someone might have to pay for?
Please don't list things that are extremely popular like the Uffizi, Louvre or even Burg Eltz. Pick something that either isn't in a guidebook or maybe only has 1 or 0 stars (triangles?) in Rick's books.
For me this would have to be the Deutsche Bahn museum in Nuremburg. It gives a complete history of the first major railway in Europe from it's infancy. It is also surprisingly and refreshingly honest about the fact that Nazi Germany took the service to the utmost heights making it the Cadillac of rail systems in the early-to-mid 20th century.