Whistle-based Authentication: http://popdevelop.com/whistle
The project uses HTML5 audio (in Chrome) together with some basic signal processing to detect notes that are whistled in a certain order. Only the order is important, so no need for perfect pitch.
And kids, don't use this in anything close to a production environment. This project was developed during the Malmoe Music Hack Weekend 2013 and only supports the two demo users. However feel free to fork the project and add your own users and melodies.