Dichlorodifluoromethane

Dichlorodifluoromethane (R-12), usually sold under the brand name Freon-12, is a chlorofluorocarbon halomethane, commonly known as CFC, used as a refrigerant and aerosol spray propellant until its manufacture was discontinued and banned of new production in the United States and many countries in 1994, due to concerns about damage to the ozone layer. It is soluble in many organic solvents.

Usage as an aerosol
The use of chlorofluorocarbons as an aerosol in medicine, for example: USP-approved Albuterol, has been phased out by the U.S. Food and Drug Administration. The use of a different propellant known as hydrofluoroalkane, or HFA, which does not harm the environment has been chosen as the replacement. 

Retrofitting
R-12 was primarily used in automobiles produced prior to 1994. For older vehicles retrofits to R-134a (1,1,1,2-tetrafluoroethane) are generally recommended. Retrofits usually include a complete flushing of the air conditioning system to remove the oil. The oils used for R12 and R134a are not generally compatible, although some oils designed for conversion to R134 are advertised as compatible with residual R-12. Additionally a new compressor, expansion valve, and dryer may be needed. In some cases, all new rubber hoses are needed. These components are usually aftermarket products which are certified as R-134a compatible. Some car manufacturers offer OEM replacement options for older R-12 air conditioning systems.

Alternative refrigerants
Below is a list of known "drop in" alternatives to R-12.
 * Freeze12
 * R-416a
 * R-414B
 * R-409A
 * R-406A
 * HC-12a

It should be noted the EPA does not refer to these as "drop-in" alternatives to R-12. The EPA has a section called "Misleading Use of Drop-In to Describe Refrigerants" which can be read here.