One of the earliest large-scale prefabricated housing projects in Finland was the Pihlajamäki district in Helsinki, built mostly between 1959 and 1965. See e.g. this StreetView. Many of the buildings have protected status today.
By far the most famous architectural example of a planned 'garden city' district of this period in Finland is Tapiola in Espoo, just outside of the border of Helsinki. The buildings of Tapiola were designed by the top architects of the day (Aarne Ervi, Viljo Revell, Aulis Blomstedt etc.), so they're not exactly an example of the nasty prefabricated blocks that followed in the 70s and 80s in many places. The center of Tapiola is currently being rebuilt to a large extent. The construction projects include a metro station, one of eight along the western metro extension that is to be opened in 2016.
If you're looking for lots of 70s and 80s prefab-fantastic construction, try the suburbs along the Vantaankoski track (M train), e.g Myyrmäki (StreetView) or Martinlaakso (StreetView).