Information on Standard Carrier Alpha Codes can be found at NMFTA.org and SCACCodeLookup.com. A good list of common carriers and their corresponding SCACs is available on EDIProvider.com.
The National Motor Freight Traffic Association's website provides some basic information on SCACs such as who they are assigned to and when the system was developed. NMFTA created the SCAC system in the 1960s to accelerate computerization of data and records in the transportation industry. SCACs are implemented in the structures of various U.S. government agencies including the U.S. Bureau of Customs and Border Protection and the Food and Drug Administration. Certain codes are reserved for specific purposes. Codes ending in U, for example, are assigned to freight containers while codes ending in X identify privately owned railroad cars.