Talsinki
Tallsinki (sometimes also Hellinna or Hellinn) is a colloquial name for the frequent, cheap and easy travel between Helsinki, Finland and Tallinn, Estonia. It is an obvious portmanteau of the cities' names. There is an official Euroregion called Helsinki–Tallinn Euregio.
"Tallsinki" is often used to imply that the commercial centre of Tallinn has become "the Helsinkians' living room". Every summer, thousands of Helsinkian tourists travel to Tallinn and back the same day, spending their entire time in Tallinn shopping in the various department stores and markets in the city centre and the Old Town, attracted by the low prices. Very few tourists venture outside the centre, and fewer still travel to other Estonian cities.
The converse happens much more rarely. Many Estonian tourists like to visit Finland every year, but the shopping phenomenon on their side is not necessarily "cheap" shopping but rather "different" shopping - it is about finding different goods. After the fall of the Soviet Union in 1991, Estonian living standard (although still not on the level of Finland) has improved drastically. In fact, many goods are more expensive in Estonia than in Finland (e.g., some food supplies, clothes, etc.)
There are also have been proposals to connect Helsinki and Tallinn as one metropolitan area, like Copenhagen and Malmö (Oresund Region), by a railroad beneath the Gulf of Finland. However, these proposals haven't got any significant support yet.