UPDATE: March 20, 2014: Horscheck has informed me that this tip is out of date and that the Euro is the currency now in use in Estonia! The Euro was adopted by Estonia in 2011.
At the time of our visit the following information was correct but it is no longer the case:
(2005) When you visit Scandinavia and the Baltic, you will find that you may need to carry many different kinds of local currency. Denmark and Sweden each have their own "crowns" or kroner; Russia still trades in rubles; Germany and Finland now have the Euro; and Estonia has Kroons (100 senti = 1 kroon), which were re-adopted after the demise of the Soviet occupation. Multiples of kroon are called "krooni."
I charged most of the larger items I purchased as credit cards were taken in most establishments; some people selling crafts in one of the passages did take American dollars, but on the whole, purchases were expected to be paid for in Estonian Kroon. There are several cambios just inside the Viru gate, and a Hansabank is also located on Viru. When we visited in August, 2005, the exchange rate was about 12 to 13 kroon per US Dollar. The best rate will probably be from a bank. If you go to a bank, be prepared to take a number and wait to be called for service.
ATMs there are plenty of bankomats in Tallinn. Most of them accept Visa / Master / Maestro Card.
A very informative Web page* is: www.balticsworldwide.com
Wear some good shoes if you wanna do a lot of walking in Tallinn. Otherwise your feet will feel the cobblestone pavements of Old Town at the end the day.
Prices in Old Town are higher than in other areas of Tallinn. Like in many places tourists have to pay a "bonus". Expect to pay 4€ for a beer (instead of 1,20€ in non tourist areas.)
At the beginning of summer the sun is nearly not going down. These days are the famous "White Nights. There are many events and its a special time! On the other hand your internal clock might be confused. (At least I felt quite bollocksed)