"On the internet no one knows you're a dog," mused one canine to another in a famous 1993 New Yorker cartoon. Today, though, thanks to techniques ranging from Web browser "cookies" to sophisticated data mining, they may know you're a dog-and even which breed.

Companies can combine information voluntarily submitted by users with data automatically transmitted by a user's Web browsers and other software to provide a detailed picture of an individual. In some cases, interested parties can discover detailed personal information about people who visit their Web sites or use their software.