If Greece runs out of Euros, Greeks will just create their own money.

According to this Telegraph article, “Businesses in Thessaloniki and other parts of the country are already creating parallel private currencies to keep trade alive and alleviate an acute shortage of liquidity.” That’s not the first time that’s happened.

In Ireland there have been two bank strikes since WWII during which the country ran short of official money. (That was prior to Ireland joining the Euro). What people did was to use cheques and endorse them and pass them from hand to hand.

Same thing happened in the initial stages of the Arab Spring two or three years ago. That is, banks closed, so people started creating and exchanging their own IOUs.

And at one stage in the 1800s there was a shortage of government issued coins in Britain. So British metal bashers turned out their own coins!

Of course that sort of unofficial money is nowhere near as efficient as money created by central banks or large commercial banks. In fact it is so inefficient that that sort of unofficial money possibly does not count as money. But it’s better than nothing. 

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P.S. (7th July 2015). There's a bit more about the Irish bank strikes in this Financial Times letter.