You don't seem to be able to buy salt to keep your sidewalks ice- and snow-free anywhere in Washington, DC these past couple of days since more prepared people snapped it all up. That, in turn, laid the groundwork for my discovery that many people don't seem to realize that ordinary table salt can perform the snow-melting function just fine. I assume the ice marketing specifically for snow-related purposes is chemically different in some respect from the table stuff, but not the relevant one.

Saltwater has a lower freezing point than does unmixed water. Indeed, the idea of the farenheit temperature scale is that zero degrees is supposed to be the freezing point of an equal mixture of salt and water (100 degrees is supposed to be human body temperature, but it got miscalculated) so the salt effect can be quite substantial if you have enough of it.

UPDATE: Yes, as they're saying in comments, rock salt is considerably cheaper. I'm just saying that if all the stores in your area run out of rock salt, and you want to melt some ice, table salt will work.