Last weekend, I built an application to track emotional and mental state using an Emotiv headset to overlay realtime data onto the image of the person. When a smile is detected, the application takes a screenshot of the page being viewed and overlays it with the image of the person smiling.
With more work, emotional and mental state could be shared in a more intuitive/creative way. Imagine a skype call were you see the persons level of excitement/boredem via some graphical overlays. Additionally, it could be used to track mental state and capture meaningful moments when working.