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.
Mergent Online
Provides company information, competitors, financials (current and historic) products, executives etc. Information can be downloaded into Excel for further analysis.
ReferenceUSA
Specializes in private company information. This is great for use in locating resources, generating sales leads and creating supplier/customer data for local markets.
OneSource
Excellent database for researching foreign companies. Contains information such as company description, historical financials, analyst reports, SWOT analysis and more. Find research on a particular company or build a list of global companies that meet your research requirements.