Eurozone

There used to be a lot more currencies in Europe than there are now, with few shared amongst countries back in the 1990s - the French franc, the Spanish peseta, the Austrian schilling...to name a few.

I'd like to imagine that this would have probably made tourism and trade across Europe more frustrating, what with all the currencies having to be exchanged for others. 

Following the establishment of the Eurozone in 1999, however, twenty currencies have been replaced over time, with many EU members joining the new zone. In total, twenty currencies have been replaced since then by one general currency - the Euro, which can be used anywhere you go within the Eurozone. Initially made up of eleven countries, that list has grown, with Croatia this year the latest to ditch their currency in favour of the Euro.

Not every country in the EU obviously uses the Euro; Denmark has an opt-out, enabling them to continue using the krone (and so did the UK when they were in the EU, enabling them to continue using the pound sterling). Six countries - Bulgaria, Czechia, Hungary, Poland, Romania and Sweden - are obliged to eventually join the Eurozone, yet only Bulgaria is in the ERM II, which sees their currency pegged to the Euro anyways. And five/six? countries - Andorra, Kosovo, Monaco, Montenegro, San Marino and Vatican City - use the Euro anyways.

Interestingly enough, the Euro isn't the only currency shared by many countries - though it is certainly the largest and most well-known and important. The West African CFA franc is used by eight countries in (unsurprisingly) West Africa and is admittedly pegged to the Euro. Six countries in Central Africa use the Central African CFA franc, also pegged to the Euro. There are also plans for a new currency, the Eco, which would effectively replace the West African CFA franc and would include even more countries.

Three Overseas French Territories in Oceania use the CFP franc (notice the pattern?), also pegged to the Euro. These three have all been around since 1945, when the countries/territories in these currency unions were French colonies. 

The East Caribbean Dollar is used by eight countries/territories, having originally been put in place in 1965, and is pegged to the US dollar. 

And that's not forgetting other interesting cases of countries sharing currencies, such as Bosnia and Herzegovina using the German Deutsch Mark in the 1990s, and Zimbabwe accepting loads of different currencies after financial difficulties and hyperinflation in 2009.