The American Business Information United States Business Directory is one of the most popular sources for looking up U.S. businesses. The ABI entries give you good basic information about an impressive number of companies, which is particularly useful for researching small companies when not much else is available. The ABI Directory is published by infoUSA (formerly "American Business Information").
ABI entries are similar to entries in the D&B Million Dollar Directory, except that the ABI provides listings for individual locations, not just the whole company. Generally, you get the company name, address, phone & fax, industry, sic code, number of employees, sales, and the top exec's name.
ABI entries are searchable on Lexis (BUSREF;ABIUS) and Dialog (File 531).
There is a Canadian equivalent call the American Business Information Canadian Business Directory, which is also available on Lexis (BUSREF;ABICAN), Westlaw (CANBUS) and Dialog (File 533).