Mostly technical grade sodium chloride, which is cheap. However, a cyanide compound is added to retard clumping and calcium chloride is substituted when the temperature is too low for NaCl to work properly.
I recall one exception. In 1958 (or thereabouts), New Orleans got its first snow in almost 70 years. It was just a couple of inches, an amount which would have been just a minor inconvenience in the North and would have been cleaned up in a couple of hours. It crippled the city. The only salt available was table salt. It worked but was a rather expensive solution.
