This is definitely one of the coolest uses of Google+ that I have seen in a long time. Hangout Quest is a Google+ game that allows you to go on a virtual scavenger hunt inside the Palace of Versailles. The object of the scavenger hunt is to find artwork and other objects in the palace. If you invite others to your Hangout you can compete against them in a race to find the objects first.
Hangout Quest uses the Street View imagery of Google Maps to bring you inside the Palace of Versailles. Another cool piece of technology added to Hangout Quest is facial tracking. The facial tracking technology allows you to move around in the Palace of Versailles by just moving your head instead of clicking around with your mouse.