That enables Internet companies to offer everything from online stock quotes to unlimited email storage for free, says Anne Toth, Yahoo’s chief trust officer. Without sophisticated advertising technology, more websites and services could wind up behind pay walls, companies warn.

The problem, argues Jeff Chester, executive director of the Center for Digital Democracy, a privacy group, is that many consumers don’t know they’re being tracked. And even if they do, they have no idea what happens to their information — whether it is used to create personal profiles, merged with offline databases or sold to data brokers — and no practical way to stop the data collection.

With growing alarm in Washington, a coalition of industry trade groups— called the Digital Advertising Alliance — has established a self-regulatory program that places icons inside the online ads of participating advertisers, ad networks and websites. The icon links to a site that explains online targeting, and lets consumers install an opt-out cookie if they just want standard ads.

Among the groups participating in the alliance are the Interactive Advertising Bureau and the Direct Marketing Association, as well as individual companies including Google and Yahoo.

Even so, these efforts don’t go far enough for the FTC. While the agency has not endorsed any particular Do Not Track technology, it believes one promising approach could involve including a setting inside Web browsers. Now the browser companies, led by Microsoft and Mozilla, are responding with different approaches:

— Microsoft has a feature called “tracking protection” in Internet Explorer 9.0 that lets users create “black lists” of Web sites to be blocked and “white lists” of sites that are deemed acceptable. Users can set their browsers to automatically build these lists or can download existing lists.

— Mozilla has a setting in its Firefox 4 browser that sends a signal to alert websites, advertisers and ad networks if a user does not want to be tracked.

Apple is expected to include a similar feature, called a “header,” in its Safari browser. Microsoft, too, recently added the feature to IE 9.0.

— Google’s Chrome browser is piggybacking on the Digital Advertising Alliance by offering a plug-in that saves opt-out cookies even if other cookies are erased. One criticism of the industry program is that users lose their opt-out preferences whenever they clear their cookies.

For such tools to work, however, there must be industry consensus on what Do Not Track obligations should actually mean. And right now, there is little agreement.