In Finland, there are both representative offices of major European car rental firms (Hertz, Avis, Sixt, Europcar, Thrifty and others) and local companies (Scandia Rent, Netrentm, Transvell Au). Their addresses can be found on their official websites or under Autovuokraamoja (Rent a sag) in telephone directories.
Car rental in Helsinki is relatively cheap - for example, a Citroen C1 or Opel Corsa costs from 27 EUR per day, a Mercedes A from 52 EUR and up. You can even rent a minibus - in this case you will pay by the hour, about 5-10 EUR per hour.
Inexpensive car rental in Helsinki is also available on weekends - the weekend package starts at 16:00 on Friday and runs until 9:00 on Monday.