The largest U.S. websites are installing new and intrusive consumer-tracking technologies on the computers of people visiting their sites—in some cases, more than 100 tracking tools at a time—a Wall Street Journal investigation has found.

The tracking files represent the leading edge of a lightly regulated, emerging industry of data-gatherers who are in effect establishing a new business model for the Internet: one based on intensive surveillance of people to sell data about, and predictions of, their interests and activities, in real time.

The Journal's study shows the extent to which Web users are in effect exchanging personal data for the broad access to information and services that is a defining feature of the Internet.

In an effort to quantify the reach and sophistication of the tracking industry, the Journal examined the 50 most popular websites in the U.S. to measure the quantity and capabilities of the "cookies," "beacons" and other trackers installed on a visitor's computer by each site. Together, the 50 sites account for roughly 40% of U.S. page-views.