Though I mostly try to actively add to my UNESCO collection, I am always very happy when a kind Postcrosser surprises me with a World Heritage site on an official card. Recently, I received a card from Germany and discovered a new UNESCO site, Quedlinburg:
The town of Quedlinburg is special because it escaped bombing during World War II and so it kept many of these original half-timbered houses from the 14th century. I had never heard of Quedlinburg before, and it was very neat to discover a new UNESCO place. So many of the sites are world famous landmarks, but it is a treat to discover a hidden gem.