It is called "Het Wilhelmus" and being over 400 years old is the oldest national anthem in the world, though it has only been the official Dutch anthem since 10 May 1932. It is thought to have been written back around 1572 but the writer of the piece remains uncertain. There are two theories, one being Philios van Marnix and the other Dirck Coornhert, but many doubt that either of the men wrote it due to the nature of the language... The origins of the tune is less uncertain and is based on a french song, "Autre chanson de la ville de Chartres assiégée par le prince de Condé".
The song tells the story of Willem van Oranje and his fight against the King of Spain and is written as if Willem himself is singing it.
Het Wilhelmus is made up of fifteen verses and is an acrostic, with the first letter of each verse originally spelling Willem van Nassov (old Dutch for Willem van Nassau).
For those of you who feel the need to stand up and sing the first verse of the Dutch national anthem, here it is:
Wilhelmus van Nassou ben ik, van Duitsen bloed,
den vaderland getrouwe blijf ik tot in den dood.
Een prinsje van Oranje ben ik, vrij onverveerd,
den koning van Hispanie heb ik altijd geëerd.
And here's how to sing it properly.