Finna.fi is a free website where users can search the material from The National Archives of Finland as well as other libraries and museums with a single search.
“Finna.fi provides free access to material from Finnish museums, libraries and archives. You can browse and read material available on the web. You can also renew loans and reserve material from various libraries in one place. Finna.fi is your one stop destination for searching the freely available material provided by the partner organisations.”
“Finna is continuously evolving, and it is gradually replacing the user interfaces of all Finnish libraries, archives and museums. At the moment, 96 organisations are involved in Finna.”
Finna has a nice feature where you can scroll to the bottom of their homepage and select a time period, hit search, and see what records are accessible for that time period.