I've been playing with a free iPhone application called Loopt (Web site, App Store). It allows you to stay connected to your friends, colleagues and relatives who are also members of the service. Loopt uses the GPS chip in the iPhone 3G and Wi-Fi triangulation in the iPhone 2G to pinpoint your exact location.
When you invite friends to join Loopt they get access to your location and can ping you when they're in the area. If this scares you remember that a) it's by invitation only, and b) you can turn off the location feature at any time.
If you travel to a different city or even one of your old haunts you can fire up Loopt to see if any of your friends are in the area. You can also use Loopt to find local places and events recommended by friends and services such as Yelp.
It also works across multiple carriers and devices so you can also share location with friends even if they don't have an iPhone. I'm currently using Loopt to track my trip on the Amtrak AutoTrain and it's pretty cool. Think of it as a location-based version of Twitter.