In the high season (June to August) many ferriesof different companies serve the route between Helsinki and Tallinn.
Please keep in mind that depending on bookings the ferry timetables might be changed on short notice. Ferries might either be cancelled or departures might be put off.
Make sure that you always check your ferry departure well in advance.
I was trying to find a ferry leaving Tallinn for Poland or Germany. I failed and was told the only option is to travel first to Finland and catch a ferry from there! :-(