These are some databases with which to start conducting company research. This page also has tips for doing this kind of investigation, and some tutorials.
Mergent Intellect is a database with global company information.
ReferenceUSA is ideal for conducting company research on a local U.S. level.
Orbis is recommended if you are trying to find financials for private companies in Europe.
Use PrivCo for U.S. private company research.
Be sure to use databases like ABI/Inform Collection or Business Source Complete, located under the "Databases for Articles" tab of this guide, to find out if anything has been written about these companies in magazines, trade periodicals, or newspapers.
When doing company research one should determine whether a company is public, privately held, or a subsidiary. Mergent Intellect can help you determine this. If you are doing research on a local level where there are many small businesses, use a database like ReferenceUSA to determine this.