Baltic Sea ferries
There are many passenger ferries crossing the Baltic Sea, between all countries along its coastlines. Those between Sweden and Finland are in Sweden called Finlandsbåt ("Finland boat"), while in Finland "Sweden boat".
Most of these ferries carry car traffic between countries. They also function as cruise ships for short shopping trips, from 2 to 48 hours. The 24-hour cruises are wild party cruises, with heavy drinking and crowded dancefloors, especially during weekends and holidays. The longer cruises tend to be more laid-back, with a larger portion of travellers who actually intend to visit a foreign country.
If you have a day or more to spare in a city served by these ferries, consider a cruise. That would give low-cost accomodation, as well a one-day stop in a new city; many of them being interesting Old Towns, with common history of the Hanseatic League, and the Danish, Swedish and Russian Empires.
Occasional cruises are special events, such as music festivals or conventions. These might cost extra.
As of June 2011 the international websites of both Tallink-Silja and Viking Line do at last have on-line booking. Check for "Red tickets" or "Last minute offers" for cruises reaching Stockholm.
Note that both Viking and Tallink Silja may have a problem with accepting international credit cards. That is not a problem in practice, as you can make a booking and pay it in the port with only a token extra charge (3-5 €).
 Major companies
[add listing] Sleep
At overnight cruises, the ticket price is per cabin. A regular cabin holds four passenger, holds a standard comparable to two-star hotel room (toilet, shower, soap, towels, reading light), and is much cheaper than a hotel room in Stockholm or Helsinki; they might be the cheapest commercial accommodation in the Baltic Sea area. Cabins and suites of higher standard are also available. If the cruise includes a day stop, you have access to your cabin throughout the day.
[add listing] Eat
Most cruise ferries have several cafés and restaurants at all price levels, at a standard typical to Sweden and Finland; you know what you get, but you won't be impressed. The most famous one is an all-you-can-eat Nordic buffet/smörgåsbord at around 30 €. Should traditionally be eaten as seven servings; herring, other seafood, cold cuts, warm meat, sausage, cheese and dessert.
[add listing] Drink
Drinks in the bar are slightly cheaper than in regular Swedish or Finnish pubs, with cocktails starting around 8 €.
[add listing] Buy
Most cruise ferries between Sweden and Finland make a stop at Åland, to earn the legal opportunity to host duty free shopping. These stores, with alcohol and tobacco prices far below Swedish and Finnish levels, are the main attraction for many passengers. Consumption on board is officially not allowed, but happens all the time. At weekend party cruises, the shops are usually shut down.
The stores also offer the classical duty-free supply of perfume, make-up and sweets. Some ships also have shops for clothing, toys and other consumer goods.
Ferries accept credit cards, and local currencies (in most cases SEK and Euros).
[add listing] Do
Gambling at slot machines and roulette tables. Social dancing to live music. Nightclubs. Most cruises have playrooms for children, and child activities. Sunbathing at the upper deck, if the weather allows.
[add listing] See
 Stay safe
To make an understatement, drunkenness is common on these ferries during night-time, and differences in languages and manners often provoke conflicts. While security guards tend to forgive drunkenness itself, they punish violent passengers by locking them up in a detention cell for the rest of the journey, or even marooning them in a foreign port.
Cabin parties are common, but security will respond when guests in the cabin area disturb the neighbors.
Avoid the open deck during night-time and harsh weather. Smoking can be done within designated compartments.
 Get around
Most cruise ships make an 8-hour stop in each city, enough for visitors to make a short tour.