ZabaSearch allows you to input the full name of any U.S. resident and produce a listing of information about that person that's a matter of public record. Addresses in the past 10 years, phone numbers, property values, you name it. For a fee, people may request checks on criminal records, bankruptcy, expanded (20-year) address history, neighbours, home value and details, aliases, some others.
This is not the first time such a service has become available online, [Wired] but it certainly looks to be incredibly comprehensive. Snopes says it's not an urban legend, but it should be noted that David ordered a comprehensive report for himself and says that a lot of the information was outdated and some of it was just automated guesswork. An article at FindLaw suggests that this is 100% legal. There's also an NPR broadcast on it here.
It is possible to opt out of it. However, this SFGate article speculates that the process is meaningless. I kind of agree. If something is a matter of public record, then it is accessible to the public. If Zabasearch doesn't provide the service, there are other sites that do (often for a fee), and if someone is really determined to find out things about you that are a matter of public record, they can do it even without the Internet. Zabasearch's website suggests methods to make your information less publicly available.
ConsumerAffairs notes that one of the creators of ZabaSearch not only had connections to the Heaven's Gate cult, but also violated his probation for an Ohio grand theft auto conviction in 1991 [the link in the article has since become defunct].
I'm going to be doing some research into Canadian law on what's a matter of public record and what isn't -- links/info appreciated if you've got them :>