As the name suggests, TwinsOrNot uses a Microsoft facial-recognition algorithm to analyze photos of two people and determine whether or not they look like twins. It's a pretty simple service, but it's considerably less likely to get the Internet all worked up than a site than a site that tells them how old they look.
There is still much fun to be had with the new tool, though. For example, TwinsOrNot is an easy way to evaluate potential celebrity doppelgangers from past centuries.
Give TwinsOrNot a try and see if you can find any long-lost relatives or distant celebrity gene-pool-mates. We're sure the service is accurate and can't imagine that Microsoft's algorithm would lie to you about that type of thing.