Apple’s newest version of iPhoto has a facial recognition feature. After you identify a few photos of your friends, it begins guessing on other photos. It picks out the faces in a photo and will ask, “Is this Chris Reinstatler?” This allows you to fine-tune the recognition. After it knows who the people in your photos are, it can assemble on-the-fly photo albums of your friends from multiple locations, sessions, film rolls, et cetera.
You may recognize the photo above from when we ran into a guy who looks like a budget version of my brother. iPhoto was not fooled! It picked out the tiny photo of my brother on the iPhone Iida is holding on the left, but it did not recognize the imposter on the right.