Google latitude is an interesting app available at least on Nokia phones that allows you to see where your friends are according to their mobile phone. At the moment it’s limited just to your gmail friends but expand to include more.
What makes this application interesting in the near future is that as more of the early adopting friends of yours install this app you’ll see which city they’re in quite easily. If the friends are in a public space then you can get more accurate directions.
It’s an interesting application.