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.
ReferenceUSA
Great tool for finding information about private companies located within a certain geographic area.
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.