Imagine that a company set up a mall store where, instead of selling trinkets, it sold information about customers' behavior. For a few cents, it could tell a saleswoman at one shop that the person who was about to walk in had already stopped at a competitor and was looking for red shoes.

That is the idea behind two new Internet companies, BlueKai and eXelate Media. They do not sell products or ad space but information about Web site visitors.

Data houses have existed in the offline world for years. When you receive a catalog in the mail, seemingly at random, it is usually because the vendor has bought data about you -- income level, interests, age, gender -- from a direct-mail marketing company.

In the online world, where data is highly specific and immediately available, directing ads based on data can be even more detailed and lucrative.

"People are realizing that it's the data that drives the value," said Omar Tawakol, the chief executive of BlueKai.

BlueKai and eXelate work in similar ways by tracking each visitor to a Web page and storing that information on the visitor's computer through a browser cookie. When someone does a search, for example, on Kayak.com for first-class flights to Paris in September, that information can be captured, and Kayak.com can sell that information using eXelate or BlueKai.

A buyer would want that information so it could cut down on wasted ads. Sure, Hilton Hotels could blanket sites with its ads, but it would rather show an ad to someone who has recently searched for flights to Paris.

"When you see a cookie on that user, you can show them travel to Paris, even though they may be on MySpace," said Mark Zagorski, the chief revenue officer of eXelate.

Savvy consumers can refuse the cookie or erase it, but some...