Traveler's Quest

Traveler's Quest is an oddity in the iOS world -- a massively multiplayer single-player game. You compete mainly against yourself (although there is Game Center ranking) to find buried "treasure" (and bury it yourself) in your neighborhood and wherever you might roam. The game makes finding hidden items easy, but if you're the one burying the treasure, you'll get gold the longer it remains hidden. While the goods are virtual, the locations are real -- and tie in with Google maps in a very innovative fashion. It's a wonderfully addictive game, but suffers greatly if there aren't many people in your area playing, since you rely on them to bury items in the first place. (Bots bury treasure occasionally, but it's much more fun when real people do so.) If you're fortunate enough to live somewhere that the playing population is thick, though, you can lose hours searching for the virtual goods.