Foursquare

Foursquare is a location based service. If you have 3G mobile phone, download the applicant and then you can use it.
Twitter told people what you are doing, Foursquare told people whereabouts. The core service of Fourquare is "check-in". And then the system will tell your friend where they can find you, and also tell you the entertainment place or relative information you can find nearby. And Foursquare make a rule to give praise to most frenquency user.
People can set their location by themselves. And one fun technology(" Layer", annouced by the third applicant company) is that you can take a photo of your location, and through picture layer transcation, the database would screen out all the information you need nearby your location, such as restraunt and other entertainment place.

Location based service is new but stands for a future trend.  And it is a good m-commerce way.