Organic Maps is one of the few applications nowadays that supports 100% of features without an active Internet connection. Install Organic Maps, download maps, throw away your SIM card, and go for a long trip on a single battery charge without any byte sent to the network.
I discovered some time ago that it is another Estonian piece of tech (well, at least the org behind it is). Maybe that is why it works so well in Tallinn, but indeed it’s great!