foursquare App
Great as a city guide, great for spotting special offers ... The game element is one thing, and it is fun to jostle for position with your peers, for checkin points and badges, but the ramifications for businesses, especially in the retail and licensed trades, are really excellent, too.
The platform continues to improve, the network is outgrowing Twitter (at the same stage of life), and users are amongst the most-engaged online. If you get lost ... someone will help. Just ask!