As the industry takes a look at refrigerants beyond HFCs, a lot of attention has been focused on CO2 (carbon dioxide). Another refrigerant that has been looked at over the years is R-290 - propane. Its flammability has kept it pretty much out of use as a refrigerant in the United States, but it has found applications in other parts of the world.
It still could find its way to North America in some applications. The most recent "Refrigerant Report" (12th edition) from Bitzer International offers a detailed look at R-290 from the perspective of contractors and technicians.