Ukrainian zen
The idea of the app appeared in 2014 year in Ukraine when the military actions started on the east of the country. Hryvnia (country currency) become unstable and the exchange rates were changing couple times per day. Every citizen were shocked and started to buy Euro and/or US Dollars. So, there was a need in rapid launch at that time.
Zen - is a great case, when app should qualitatively do only one useful function and users will be really happy.