Built in two weeks by US media agency UseAllFive as part of a Google+-centric hackathon, Hangout Quest charges players with locating artworks inside various famous museums. It combines Google Hangouts and Streetview, with the result resembling an online, multiplayer version of Carmen Sandiego.
Of course, it's not quite the same — you're not chasing down criminals or anything — but you do have to locate hidden clues and use those to find "treasures" within each location. The idea is to play against friends, collect as many points as possible and show everyone just how much of a culture snob you are.
While I doubt it will get anyone excited about using Google+, it is a neat example of how the numerous public APIs for Google's services can be used together.