Companies Register – WW- aggregation of companies registers worldwide, maintained by UK Companies House.
Data Axle- key information about North American companies, often verified by telephone calls; now more focused on being a sales targeting platform than a research tool.
D&B Hoovers- subscription database from Dun & Bradstreet with information about companies worldwide; huge database but now more focused on being a sales targeting platform than a research tool.
Footnoted- exciting findings from the small print of annual reports; Footnoted follows the clues in the back pages that most of us never read.
Mergent Online- aggregated information for US and global companies, with features for creating custom data tables.
Open Corporatesprovides legal entity information for companies worldwide. It is more focused on a company's structure and key officers than its financials.
SEC—The SEC's database of annual reports and other filings is a must-have for any competitor analysis of US-listed companies.
Trefisis a data analysis tool with a unique perspective on how public companies' individual divisions/products contribute to the company's stock price or market capitalization.