There's an interesting article from last year over on Macworld about how spammers try to use comment systems on discussion forums to promote their wares.
Christopher Breen, one of their moderators, dug into the world of Chinese companies promoting dodgy (as in "don't work very well") video converters. "The software is advertised through forum spam -- usually in the form of tutorials that show you how to use it -- but there's sock puppeting going on as well," he says. "These companies will often first pose a question along the lines of 'How do I convert my videos for the iPad?' and then, under a different identity (but using the same IP address), answer their own question with a recommendation for one of these tools. Additionally, they create shareware sites that, coincidentally enough, rave about these products in fake reviews."
Breen managed to confront some of these spammers when they contacted Macworld asking them to review their software. "I was contacted by a PR representative from one of these companies asking for a review. I replied that we don't review software from spammers but if they were willing to forego this sleazy practice, we might consider a review, just as we'd consider a review of any legitimate product."
He goes on to detail how the spammers offered to pay for advertising and offer special deals and prizes to readers in return for a review, and he always replied, "We don't support spammers."
Breen lists the names of many of the video converters to avoid and offers advice to those running discussion forums on how to recognize and remove their spam.