Hoover's Online
Search for information on both domestic and international companies. Contains data about competitors, financial information (up to 10 years worth of historical financial data for public companies), history/background, primary products etc. Very easy to use to find a single company or generate a list of similar companies.
Find data about businesses within an area from large corporations, to single-employee operations. Provides basic business statistics for all companies like location sales, revenues, employees and expenditures. Excellent source for private-company data.