Capital IQ - comprehensive fundamental and quantitative company research and analysis. Note: requires registration.
Onesource - contains US and international, public and private company and industry profiles.
Privco - provides detailed business and financial data on major, non-publicly traded corporations, including family owned, private equity owned, venture backed, and international unlisted companies.
ReferenceUSA - searchable database with detailed information, including addresses and phone numbers, on more than 12 million U.S. businesses, 102 million U.S. residents, 683,000 U.S. health care providers, and 95,000 Technology Companies and 280,000 Executives.
Mergent - Contains financial details of over 14,000 active and inactive U.S. public companies listed on the NYSE, AMEX and NASDAQ exchanges.