Euro
The lawful currency for the time being of the European Union, even though not all members of that august association use it — those outside the Eurozone don’t, and some cheeky chappies outside the European Union do use it. Like chippy little Montenegro, down on the Balkan coast, snuggling next to EU member but euro refusenik Croatia.
Opinion divided, after twenty years, whether it was a good idea or not: but it has managed to hang on, and maintain a relatively stable value, notwithstanding the global financial crisis, the Greek currency crisis, and that excruciating Anglo-Saxon exercise in will-he-or-won’t-he, Brexit.