A couple of days ago a new service was launched to bring old Finnish maps available online. The project is named after researcher Heikki Rantatupa who has worked with the maps for years. Thanks to his efforts the service covers about 5400 maps in 6350 pictures. Most of the maps in the service are from the years 1600-1800 but some maps exist until 1950's.
The map service is hosted by University of Jyväskylä and it is unfortunately currently available only in Finnish, but it is easy to use if you know the name of the place you are looking for. Just look for the upper corner of the search page where there is the search field witht the finnish text "Karttahaku". Type the name of the location you are searching for and you will get listing of maps relevant to your search. Link to the Heikki Rantatupa historical maps portal.