Foursquare (http://foursquare.com/).

Foursquare's ascension into the mainstream has been incredibly fast & impressive, especially given that on the surface there was little differentiation among all the competing LBS check-in services a year ago.

I think 4SQ is a good case study in the importance of the execution over the idea (a commonly harped on subject in these parts).

4SQ's success also helps to validate NYC as having some distinct advantages for certain types of social media plays.