Community-based spam filtering leverages the most finely tuned spam detection device in the world—the human brain—and relies on the fact that spam, by definition, is sent to thousands of victims. When enough community members mark a message as garbage, nobody else in the community has to look at that same junk mail. SPAMfighter Standard implements this technique effectively, and it's free for personal use.

You do pay in a way: The program adds a SPAMfighter footer to your outgoing e-mails. You may not mind seeing the ads, but your friends might not appreciate them, and it's certainly not the sort of thing you want in your business e-mails. The software also displays small text advertisements in its toolbar. Pay $29 for SPAMfighter Pro and you get rid of the ads and footer. You also gain the ability to blacklist or whitelist unlimited domains and addresses (the free edition is limited to 100). And the Pro edition can block messages written in specific foreign languages. The Standard edition runs with all of the Pro features enabled for the first 30 days.

The software supports Microsoft Outlook versions 2000 or later, Outlook Express 5.5 or later, and Vista's Windows Mail. Once installed, it filters all incoming mail regardless of the account type, placing nuisance messages in an automatically created SPAMfighter folder. And it installs a toolbar that lets you quickly block or unblock messages and whitelist or blacklist their senders.

As with any community-based spam filter, junk sometimes slips through. When that happens, do your civic duty: Click on the Block button so that your fellow community members won't get spammed. In the unlikely event that this action results in a valid message getting blocked, just click on Unblock.

Like Cloudmark Desktop and iHateSpam, SPAMfighter waits for a critical number of community members to block a message actively before marking that message as spam for all other members. The other two use an elaborate trust system to determine how much weight to give each community member's blocking actions. Briefly, the more your actions match those of the entire community, the more weight future actions will have. The aim is to prevent abuse of the system by, say, an unscrupulous business man trying to interfere with a competitor's valid customer communications. Users also enjoy seeing their trust levels rise. But the absence of this trust system doesn't seem to harm SPAMfighter's accuracy.

The blacklist and whitelist feature can override community-based filtering. Mail from a blacklisted address or domain is always blocked, while missives from a whitelisted address or domain are always allowed. You don't need this feature nearly as much as you do when using a spam filter that detects spam by analyzing content, but it's available. Like many antispam products, SPAMfighter imports your address book into its whitelist at start-up and does so again on demand. But here's a nice twist: It can also import the blocked senders list from your e-mail client into the blacklist.

Wonder how well it's working? The software's Statistics page shows how many spam messages it has blocked for you and for the entire community of 5 million users. According to this page, it has headed off over 15 billion spam messages overall.—Next: Does It Work?