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Anyone who has ever had an inbox full of penny-stock tips, pharmaceutical offers, and phishing attempts has had two fantasies: To turn off the spigot, and to find out who gave the spammers their address. Throwaway e-mail addresses can make both fantasies come true, at least for some of your spam.

With the right e-mail service, you can set up unique, disposable e-mail addresses for any transaction in which you're worried about getting spammed. The e-mail arrives in your regular inbox, but if you start to receive spam from the disposable address, you can just turn it off. In addition, the address can help you pinpoint exactly which company sold you out.

I looked at three services: Yahoo AddressGuard (part of the $20 per year Mail Plus service), Gmail disposable addresses (free), and a relatively new one called Anonymizer Nyms ($20 per year). With Yahoo's feature you choose a single unique prefix for all of the addresses you create, such as "erik-," and then append a suffix for each new address. I tend to use a site's domain name for the suffix, so an e-mail address might look something like erik-sleazycompany@yahoo.com. You can add notes about when and why you did business with the company when you create the address.

With Nyms, you select a unique name for each address by visiting the Nyms Web site or by using software on your PC. All your Nyms addresses end in "nyms.net," and mail sent to any of them ends up at whichever e-mail address you list in your account. Of the three services, Nyms offers the most features, such as the ability to specify an address's expiration date.

If you respond to e-mail sent to the Nyms or Yahoo addresses, your responses show as coming from the temporary address. In addition, either service allows you to create a new address in roughly 30 seconds with a toolbar bookmark.

Gmail's feature is more convenient than Yahoo's or Nym's in that it requires no additional setup, but it carries some significant drawbacks. To use it, you add "+anyword" to your regular Gmail address, as in elarkin+sleazycompany@gmail.com. Any such e-mail will come to your regular account.

Because you don't specifically create an address, however, you can't simply delete it if spam starts flowing. You must create a mail filter to block or delete any mail sent to the address. Also, if you reply to a message, the reply shows as coming from your regular Gmail address. And finally, smart spammers can just strip out the "+sleazycompany" and use your true address. Still, it's better than directly handing out your actual e-mail address.

I regularly use Yahoo's service because I'm also a fan of the site's new Web mail interface. But if I didn't already have a Yahoo account, I'd probably fork over the $20 for Nyms, since it works smoothly with any other e-mail account.

Erik Larkin is an associate editor for PC World. Send e-mail to privacywatch@pcworld.com.