EDGAR filings too can be searched for information about private companies, and while private companies do not have to disclose their finances or information pertaining to their business, mention of them may show up in public company filings, as in footnotes. 

All companies, foreign and domestic, are required to file registration statements, periodic reports and other forms electronically through EDGAR.  http://www.sec.gov/edgar.shtml. But not all documents are not yet permitted to be filed electronically and consequently will not be available on EDGAR.  There are numerous additional limitations, exclusions and special cases.

Federal or state law determines what is a public record. Because the laws of each state vary, what is public record in one state may not be public record in another state.

Profiles of individuals in prospect research, which is used for fundraising and development efforts or as a means to more fully understand a potential business client, partner, investor, or future manager, are built using multiple data sources such as LexisNexis, Factiva, Hoovers, EDGAR, and other sources.  Source and accuracy, as well as timeliness of information gathered must be verified, to ensure data integrity and reliablity.

Search engines:  Another new search engine is knuru, a U.K-based search engine that uses a natural language query in the form of short descriptive sentences and phrases.  knuru powers "Knowledge @ Wharton" (Wharton Business School's business content and analysis).  Searches are not a function of page rankings or other artificial commercial juxtaposition of search results.  Full text news articles are available through knuru too.  See http://www.knuru.com/.

Search engines: if you want your searching to take on a more personal and entertaining format, try out Ms. Dewey at http://www.msdewey.com.  In this format, you're able to communicate with a "person", who appears to listen and respond to your queries, and provides detailed responses, along with entertaining commentary.