Facial recognition can be a frightening technology, posing a privacy threat so great that Google has placed a ban on any Glass apps that employ it. Yet marketers have been using the mug-reading tech for years now as part of their interactive campaigns. Privacy issues aside, once people decide to opt in, facial recognition can drive some innovative, highly entertaining advertising ideas. Check out some of these efforts below.

Virgin Mobile's Blinkwashing
Eariler this week, Virgin Mobile USA teamed up with Mother New York, rehabstudio and Greencard pictures to create this interactive Youtube video whose scenes you could change by simply blinking. It combined 25 different films, leading to a total of more than two million different combinations.

Volkswagen Start/Stop
To promote Volkswagen's Stop Start technology, which turns the car's engine off when it's stationary, the automaker worked with Almap BBDO to create a Chrome app that stops Youtube videos when you happen to look away from them.

Nike Free Face
Nike and Wieden + Kennedy Tokyo invited consumers to work out their faces with NIke "Free Face." It allowed users to control the movement of the Nike Free shoe, known for its flexible sole, just by squirming their faces around. It works by combining both facial and expression recognition technology.

Almax Eye See
Speaking of expressions, consumers have to do no more than stare and react to the goods they see in a store window to get this baby going. Italian company Almax invented special "seeing" mannequins, which retailers can place in storefronts. With built-in cameras, the mannequins can help marketers gather demographic information, like age, gender and race, about those eyeing their products. Retailers including Bennetton reportedly have employed the dolls.

Hunted: Byantium Securities
This elaborate online campaign to promote the Cinemax espionage-themed show "Hunted" required visitors to take a series of tests challenging their spy skills. One, aided by facial recognition technology, tested users' ability to control their emotive responses.

Plan U.K. Because I'm a Girl
Only women were able to view this clever outdoor ad for British charity Plan UK. The bus shelter appeared on London's Oxford Street for two weeks last year and used facial recognition technology to determine if a woman or man was standing before its screen. Different content would run, depending on the gender of the passer-by. People had to opt-in to view the work but only women were allowed to see the full 40-second ad. The idea was meant to highlight how females around the world are denied opportunities and choices on a daily basis due to discrimination and poverty.

Coke Zero Facial Profiler
One of the earliest innovative ad applications of facial recognition technology, this 2009 Facebook campaign for Coke Zero out of MDC Partners' CPB promised to match people up with their doppelgangers. They had to opt in, of course, but once they did, the application tried to match them with lookalikes on the social network. Others could vote on how good the likeness was, helping to refine the system.