A good way to get to Tallinn or Stockholm - much greener than flying. Lots of services, and all ferry terminals are walking distance to city centre accommodation. However, you should book a cabin if doing the overnight ferry trip to Stockholm, as the bars, cafes and clubs are terrible, and there is nowhere quiet for you to sleep in a chair.
Viking Line Terminal;
www.vikingline.fi/index.asp?lang=en
Updates
Can't exactly agree with you there, jimwhitters. For one, the huge amounts of pollution churned out by these liners is already up for debate, but as this is the holiday and not politics section, we'll move on. In regards to finding somewhere to sleep, it IS possible if you are travelling without a cabin. If you are travelling Turku-Stockholm, the ferry has two airline seat rooms (choose the one upstairs as it is darker and quieter), while on voyages between Stockholm and Helsinki there is a bare 'sleeping room' on the 8th floor.
If you do decide to go for this option, bear in mind that it is absolutely freezing and not particularly comfortable, so bring a sleeping bag and inflatable mattress (there are lockers at all ports for €3-5) and get in there early: most the sleeping spots have been shotgunned by 10:30pm and you may find yourself out on a corridor.
Remember that is possible at present to get up to a 50% discount on ferry fares if you are also travelling with an interrail pass: the price combined for the two routes above came to only €26!
Send your feedback or queries to been.there@guardian.co.uk
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It's a great cruise taking 24 hours. Link to UK agent: www.viamare.com or to Superfast site.