It happens all the time: A person loses a cell phone, and someone else finds it and keeps it; a person wants to buy something online, but is worried that he'll be cheated out of his money; a woman walking down the street is whistled at and feels helpless.

It used to be, the only way you'd hear these stories was if you knew the person. Not anymore. People are using the Web to try and make public – in a big way – what might once have been just private matters.

Take the recent case of the missing Sidekick. When a woman left hers in a cab, instead of returning it, the people who found it decided to use it. To take pictures with it. To sign on to an instant messaging client with it. That turned out to have very unexpected consequences.

You see, all of a user's information on a Sidekick is backed up on T-Mobile's central servers (which is how Paris Hilton's information was hacked).

When messages asking for the device's return went unanswered, the original owner created a website, with the help of a friend, showcasing pictures and personal information uploaded onto the device by the new owners. The ensuing exchange of email and threats – posted on the site for all to see – is still escalating, and news of the story has spread like wildfire after being posted on Digg and numerous blogs.

In 2004, a man trying to sell his Apple Powerbook on eBay was contacted by a potential buyer, but right away the seller noticed something fishy when reading through the  buyer's proposed terms of sale. They involved signing up for an obscure escrow site and channeling the funds through it – a well known escrow scam. But rather than turning the buyer in to the authorities or eBay, the seller decided on a more public response.

He played along as if he suspected nothing, all the while posting updates of his interactions with the buyer to a Web message board. When it came time to ship the laptop to the buyer in London, the seller instead sent an old three-ring binder with ports and a screen drawn on, an ancient keyboard taped to the inside, and "P-P-P-Powerbook!" scrawled on the front cover. The seller also declared the value of the package as $2200, or the value of a real laptop, which meant the buyer had to pay almost $600 in import taxes.

That alone would have been the stuff of Internet legend, but thanks to the global reach of the Web, readers of the message board staked out the buyer's address, and were on hand when the package was delivered, reporting back to the board with pictures and posts.

More recently, an eBay seller in a similar situation used Google Maps and Windows Live Local to pinpoint his buyer's address. Seeing a parking lot and some rusty cars where the buyer said a warehouse should be, the seller got suspicious. He posted his dilemma to a Web forum, and  after further investigation found he was dealing with someone who apparently was using someone else's PayPal account to try and buy the computer. In that case, the police got involved, but the person "caught" was merely an underling - someone hired to receive and forward packages for an unknown third party.

It's easy to see the power of the Web to investigate and call attention to misdeeds. But it's much harder to remember that, too often, there's no way to tell whether something really happened, or whether a person is telling the truth online.

Take, for instance, HollabackNYC.com. It's a site that posts photos and stories sent in by readers who say they have been the victims of sexual harassment because they are women. As a concept, the site hopes to provide a kind of community service: to embarrass or shame men into treating women with more respect. But there's no way to prove the stories, and that might not be the point. Maybe the posters just want to share their anger, and feel less like a victim.

The Web is all about empowerment. These sites and stories involve people who made innovative use of technological tools and the Web's powerful ability to broadcast information, to change their status, get the word out, or stand up for themselves. As these tools – cameraphones, satellite imagery websites, and word-of-mouth sites like Digg – gain prominence, we can expect to see even more stories like them. The unanswered question, then, is whether these types of stories will be posted just to get the word out and to do the right thing, or as Web-based form of vigilante justice.