Milford Sound is a fiord found in the south-west part of New Zealand's South Island. The fiord is part of the Fiordland National Park, Piopiotahi (Milford Sound) Marine Reserve and the Te Wahipounamu World Heritage site. That should be enough to let you know the importance and cultural relevance of the place. Most people have heard of fiords in Norway, but the ones found in different parts of the world are equally majestic.
Milford Sound has been judged by many experts as one of the world's top travel destinations (according to a 2008 Travelers' Choice Destinations Awards from TripAdvisor). It has also been claimed to be New Zealand's most famous tourist destination, even more so than Auckland, the capital city. If you are ever nearby, this is a place you should definitely visit, and even try to kayak.