The Standard Industrial Classification has been replaced by the new North American Industry Classification System (NAICS), but several data sets are still available with SIC-based data. Both SIC and NAICS classify establishments by their primary type of activity.
Data available on this site by SIC:
Lookup system to find the SIC code for keywords that you enter (transfers you to OSHA web site).
The NAICS web site includes correspondence tables and a discussion of How NAICS Will Affect Data Users.
Definitions of individual SIC codes may be accessed through the first two links under 1997 Economic Census, above.