You know, that annoying phrase that you have to fill in when you register on a site, but stops all the unwanted spammers.
In this new version, the captcha is the advert, with quoted text that you have to enter, see the link to an example image below.
http://mediamemo.allthingsd.com/files/2010/09/solve-captcha-example.jpg
Ok, so its no good for clicks and conversions, but its a great potential branding exercise, or at least I think so

Solve Media Claims It Can Turn Unreadable Captchas into Ad Generators | Peter Kafka | MediaMemo | AllThingsD