|Croatia 10, 20, and 50|
Croatia's currency today is called the kuna. 1 kuna = 100 lipa. Membership in the European Union usually leads to membership in the Eurozone (the subset of EU countries which also exclusively use the euro as their currency) a few years later, although there are some exceptions (Great Britain and Denmark have special exemptions, for example).
This means that by joining the European Union, Croatia will eventually switch their money (and, more importantly for readers of this blog, their coins) from the kuna to the euro. The last countries to convert to the euro were Estonia in 2011, and Slovakia in 2009.
Of the current 27 European Union countries, only 17 exclusively use the euro. The other 10 are either exempt or are taking the steps necessary in order to eventually switch. With a stable currency, the switch to the euro can take as little as 2 years. But according to news articles, the current situation with Greece (which is a member of the Eurozone) may cause the Zone to be more cautious about allowing new members and may delay Croatia's switch to the euro until 2017.
So even though you may be hearing about Croatia joining the EU, don't expect to see any new Croatian euro coins for quite some time. The next most likely countries to adopt the euro, based on their progress through the conversion prerequisites, appear to be Lithuania and Latvia.