The U.S. Securities and Exchange Commission (SEC) requires the filing of a Form D "to offer and sell securities without having to register the offering with the SEC." This is commonly used to allow privately help companies to raise capital from venture capital and angel investors.
Form D filings are public, and available from the SEC's EDGAR database or from various third parties who attempt to make the data more user-friendly and useful, a great example of which is Rank and Filed.
Having looked through these filings, the question arises: where and how do news sources (such as VentureWire or the Venture Capital Dispatch) discover who the investors are in any given private offering? This data isn't listed on Form D as far as I can tell (only the total number of investors).