Web sites know a lot more about you than you probably realize. Mostly, site owners care about the aggregate behavior of users -- and are not out to invade your privacy. But two factors threaten what remains of privacy: First, improvments in technology may make it possible for others to identify you as an individual out of snippets of "nonpersonally identifiable" data. Second, the government's growing appetite for information about people's Web behavior might trump any site's privacy pledges