(FORTUNE Magazine) – Purveyors of diet pills and "herbal Viagra" have a new enemy. The people who made Napster the scourge of the music industry have taken on spam. Software engineer Vipul Ved Prakash and Napster's co-founder Jordan Ritter formed Cloudmark two years ago and last June began a trial phase of SpamNet, which claims to eliminate about 90% of junk e-mails. They plan to officially launch the software this quarter.

The technology, which is the most promising spam fighter so far, works like this: A consumer downloads the software (which is free in its trial phase), then hits the "block" button whenever he receives spam. After that the unwanted e-mail is rejected by any computer loaded with SpamNet software. "With Napster, the value of the network increased exponentially with the number of people who joined it," says CEO Karl Jacob. "It's the same with Cloudmark."

About 350,000 people have downloaded the technology since it became available last summer. Consumer advocate Ray Everett-Church, from the Coalition of Unsolicited Commercial Email, says SpamNet is the only filter he uses on his personal e-mail: "I'm getting only one or two messages a day that aren't being caught, and it's pretty good in terms of not identifying legitimate mail as spam." Earthlink estimates that spam has surged 500% in the past 18 months, which means there's a big market for spam-busting software--unless that "herbal Viagra" actually works. --Julie Creswell