Judging by the frenzied claims of lawmakers like US representative Jackie Speier, enabling the Do Not Track feature ranks up there with locking doors and shredding credit card statements. "People have a right to surf the web without Big Brother watching their every move and announcing it to the world," Speier said last February,when she introduced a bill to regulate online tracking.

DNT settings, which ask ad networks and the websites you visit not to track you, are the brainchild of security and privacy rabble-rouser Christopher Soghoian. His idea was to port the functionality of the Do Not Call list to the Internet, without forcing users to figure out cookies or register their computers in a national database. Firefox, IE9, and Safari all offer the option.

But none of that really matters, since the setting has no legal muscle. Websites are free to ignore it. And they do. As of June, only five had pledged to follow it—the Associated Press plus four of the hundreds of behavior-tracking ad networks whose raison d'èAtre is figuring out how you view the web.

"Companies are building detailed dossiers on consumers based on their browsing behavior," Soghoian says. "The DNT header protects you by sending a clear, unambiguous signal that you don't want to be tracked."

So turning on the feature could be like signing a petition. If enough people do it, the thinking goes, it will give lawmakers and privacy advocates more ammunition to push for change. That's just what some groups fear.

"Something has to fund all that great free online content and all those amazing digital services," says Adam Thierer, a research fellow with the Mercatus Center, a think tank focused largely on opposing market regulation. "This stuff doesn't just fall to earth like manna from heaven."

But Safari has blocked third-party cookies for years, basically the same protection a DNT asks for. "If the industry can continue to make money off of Apple users even without the ability to target them," Soghoian says, "then the industry can figure out how to survive when a few people decide to say 'Stop tracking me.'"

So if privacy is a big concern of yours, go ahead and turn on your DNT setting. It can't hurt. If you use Chrome, though, you're out of luck; Google has balked at adding the feature—after all, it does run the largest behavior-tracking ad system on the net.