Interesting.
Not free for just anybody, and not anonymous. Similar to a driver's license, where it is both license and form of ID.
"Starting this year, all residents of Tallinn can use public transport for free but they have to acquire a personalized public transport card and register it when entering the public transport vehicle. All Estonian pupils up to the age of 19 and other groups of people who have the right for free public transport have the right for free transport in Tallinn."