Large multinational corporations listed on public stock exchanges are required by the U.S. Securities and Exchange Commission to release annual reports containing detailed financial information, including subsidiary information. If subsidiary lists are not contained on corporate websites, these financial reports often are.
Privately owned corporations are not required to disclose much information and are given considerable freedom to shield information. Wikipedia maintains an extensive list of large multinational corporations with general information on each company, and in many cases lists of subsidiaries.
The principal form submitted by a public company is called a 10-K. The form offers a detailed picture of the company's business, with financial and operating information for a given fiscal year.