A master class in online snooping

Before the Internet, being a private detective was tough, time-consuming work. Gumshoes like Philip Marlowe spent days piecing together clues, getting mixed up with brass-knuckled brutes and sociopathic femmes fatales along the way.

Thanks to the Web, all that has changed. With a few keystrokes, you can uncover the names and unlisted phone numbers of the suspicious people who live down the street, find out where they lived before moving next to you, how much they paid for their house, and how generously they donated to their favorite presidential candidate. You can also retrieve websites that were taken down for nefarious reasons, surf anonymously, and easily create false identities.

And unlike Marlowe, who had to bribe every unsavory character he came across to pry loose a morsel of information, you needn't spend a penny to find what you're looking for online. The following list of free websites will yield more dirt than pressing a double sawbuck into the hand of a tight-lipped bartender.

Adapted from Rule the Web: How To Do Anything and Everything on the Internet—Better, Faster, Easier, by Mark Frauenfelder (St. Martin's Griffin, June 2007).

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1. How can I find someone's unlisted phone number, their current address, and where they've lived previously?

All of the big search engines offer some kind of people finder service, but they're little more than online phonebooks. If a person has an unlisted number, you are out of luck. But not if you use ZabaSearch. The search engine, which gets its information from public databases that aren't always directly linked to the Web, has got the goods on almost everyone. Even though many of the addresses and phone numbers in it are outdated, I've used it successfully more than once to track down someone I needed to get in touch with for a story I was writing. One such person, a well-known author, asked me how I got his phone number. When I told him about ZabaSearch, he checked out the site himself and e-mailed me back, thanking me for introducing him to this useful service.

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2. How can I see websites that were taken down for nebulous reasons?
Even though countless websites are deleted by their owners or the authorities every day, it's a simple matter to find archived copies of the expurgated sites and find out what they were trying to hide.

The Internet Archive's Wayback Machine provides a fossil record of extinct websites. To use it, enter the Web address you hope to find and click "Take Me Back." If you're lucky, the Archive will have at least one copy (or more, organized by date). Firefox users can install an extension called ErrorZilla Mod that displays a special error page when you can't reach a web page. It gives you the option to try loading the page again, or searching Google Cache or the Wayback Machine.