Based on articles like this, I predict Greece will default and will go off or be forced off the Euro.
When that happens, their new/(old?) currency will depreciate dramatically in comparison with the terms of trade they had under the Euro.
The Gubmnt will not be able to borrow except at very high interest rates, especially not until lenders have a better idea of what the current and future exchange rates will be. Forward markets will be fun, to say the very least.
Local businesses will actually (eventually) find that borrowing becomes slightly less costly because the international currency markets will allow better predictions (and hedging about) future currency movements.
Consumers in Greece will be nailed as imports become considerably more expensive.
Exporters will at first experience unsettled conditions as they and others try to figure out where the currency will move and how fast, but eventually exporters will benefit from the de facto devaluation of the Greek currency.
Other big gainers: tourists who visit Greece (my guess is that the devaluation will continue over the next year, so wait to book your holidays).
Other gainers: scammers and con artists preying on the econ-ignorant and naive tourists.
There will be considerable turmoil for several months, but eventually Greece will be forced to deal with its fiscal problems by itself as the EU, ECB, and IMF no longer bail them out. If they didn't want austerity under the Euro, they'll have it in spades after leaving the Euro. Meanwhile, they'll have to deal with their tax-evasion, pension, and cronyism problems internally.
At the same time, the Greek gubmnt will likely be forced to print more money and will monetize their debt, leading to even more inflation than they would otherwise experience. And given the left-leaning gubmnt in power, there is a strong likelihood of price controls, more currency controls, etc., a la Venezuela.
Greece will be tempted to have closer ties with Putin's Russia and possibly Iran. And threats of this realignment might lead the US to support some sort of propping up of Greece's economy.