If I am in a group in World of Warcraft, I can view a map that shows where all members of the group are. It occurs to me that the same ability would be useful outside the game—and that implementing it would be straightforward. Simply have two (or more) smartphones with gps talking to each other. Your phone tells mine where you are, mine shows me where both of us are, while mine tells yours which shows you. Useful for finding your spouse in a crowded art and wine fair, shopping mall, or equivalent.
Does it already exist?