SIC/NAICS Directories are numerical classification systems to help identify and organize like companies within the same industry.
NAICS Search The North American Industry Classification system (NAICS) is the standard used by Federal statistical agencies in classifying business establishments for the purpose of collecting and analyzing statistical data related to the U.S. business economy.
SIC Manual SIC codes and their descriptions at the Occupational Safety & Health Administration (OSHA) website
Websters Online A quick way to look up SIC codes and corresponding NAICS codes (use the left column).
Corporations also provide a wealth of information through their own websites, often under investor relations, or a similar link.
Here are some examples: