Responsive web design is one thing; but a font that changes size according to your distance away from the screen? An experiment by web designer Marko Dugonjić takes the concept of responsive typography to a new level, using face detection to adjust font size by your proximity to the display.
Using your webcam, after explicit permission is given, the software will track your face and resizee the text, based upon how far away you are. Of course it is a little buggy, being a prototype and all; but the potential makes for an exciting future of interface design.
Scaling up the subtitles of a TV show, expanding the text of a website shown on a smartphone. While it may not work on the likes of a laptop or tablet, it could be used in some pretty cool ways for larger displays or smaller phone screens.
Word of warning, you will look slightly odd moving back and forth in your office/room to try this out.