Some links I've gathered on the subject: http://www.blackhat-seo.com/2010/captcha-advertising/ http://www.blackhat-seo.com/2010/captcha-advertising/ I found 4

Some links I've gathered on the subject: http://www.blackhat-seo.com/2010/captcha-advertising/ http://www.blackhat-seo.com/2010/captcha-advertising/ I found 4 companies that are currently providing some kind of captcha advertising. It's an old idea that is being rediscovered from time to time. I don't think you need an enormous amount of words/slogans/phrases. You can use different non-linear transformations on your content, different fonts, different layouts and so on. You can also fall back on a regular captcha, either for effectiveness or to not annoy users too much.