The Greeks are somewhat the architects of their own downfall with having too generous retirement schemes etc and crucially a ridiculous tax collection system.
Having said that the IMF, Eurozone/Germany etc all knew this when they lent to them. These loans that are now due to these institutions were ridiculously short term, there was never a chance Greece could pay them back. They were the equivalent of Wonga.
And, as with all lending, the creditors should've weighed up the risk better, so it's them that takes a hit when Greece, inevitably, couldn't pay them back.
As Charlie Sheen says above, our ability to devalue our currency, drastically lower interest rates and print a shed load of money is what has primarily helped keep the crisis from being a catastrophe. Despite the bizarre situation where Monetary Policy was at complete with Fiscal Policy. Had they both been aligned then the crisis would only have lasted a couple of years in Britain and we'd be in a far better position now.
I really can't see a way out of this for Greece other than an exit from the Euro. Which would be crippling in the short term until the new currency stabilises. But I can't see the Eurozone countries (ie Germany) allowing any significant concessions to Greece, otherwise Spain, Italy, Ireland etc will soon follow. And a Greek exit from the Euro massively harms the Eurozone too.
But then they should've thought of that before allowing vastly different economies in to a currency union.