The new smartphone application called Glympse is absolutely brilliant. Where Foursquare allows your friends to see where you are, Glympse allows your friends to see where you are heading. It combines everything that has previously annoyed me about other location services such as Google Latitude.
The problem with Google Latitude was the fact that you constantly had to go online to update your location and see where your friends where. Moreover, being an iPhone user myself, I find Google Latitude annoying because you first have to enter the app, which then directs you to a web browser and then you have to log in and then finally update your location.