Android Vision API

Back in the day if you wanted face detection, you had to code it yourself. How much easier and simpler it is now, with the new Vision API on Android:

It even tells you how much the detected face is smiling! Awesome.

Arjunu.com has a great tutorial on how to integrate it into your project, check it out.

Also, you can check out the official samples on GitHub.

On a slightly off-topic sidenote (using face detection, but not the Vision API), I just saw Boo! on Android Experiments yesterday: 

Little creatures gather on the screen. As soon as they see your face, they flee in panic. Too cute to not include it here :)