The Gärten der Welt (Gardens of the World) or Erholungspark Marzahn is in the Marzahn district of Berlin. Not exactly the most beautiful part of the capital (it's still dominated by the "modern" architecture of the former communist regime), but the park is definitely worth a visit.
It opened in May 1987 as the Berliner Gartenschau and encompasses more than a 100 hectares (250 acres). The communist dictatorship tried to gather some international goodwill and got an almost free park in return. You have to pay an entrance fee nowadays, but you'll be busy walking around, looking at the landscapes, architecture and flowers for at least half a day.
In the park you'll find a Chinese, a Japanese, a Balinese, a Korean, an Italian, and an oriental garden and some extra cultural additions (like a fairytale park) to those.