Now *this* is neat; basically, people create virtual sticky notes for a set of GPS coordinates, and then leave them there for anyone else with the same system to find.
Imagine walking around a city with an entire city's worth of memories about a particular spot. You walk to a spot with no particular memories, and find loves won, loves lost, joy, heartbreak, and anger all in one spot that you never knew about.
Or walking about to find some treasure map... and the only clue is knowing where the right location to stand is.
"Bob stood on a spot accidentally, at just the right time of day, and suddenly his sticky pad rang. He turned it on, and read the following message: 'It's been 10 years. Follow my directions exactly.'"