Search PrivCo for Private Company information.
In addition to using the UI fee based sites below you can also read a company's web site and gather info from the free sites on the web.
Keep in mind that you'll find more information on a public company than a private company. Public companies sell shares to the public and as a result have to disclose financials and detailed operations info to the Securities and Exchange Commission. This makes it easy to find info such as financial statements and executive compensation. Private companies are privately owned by individuals or groups. Privately owned companies do not have to disclose info to the SEC. As a result, it can be difficult to find a great deal of information on a private company.