Google's released Latitude, a Maps tool that allows for automatic tracking of friends in real time, using a laptop, Symbian 60, Blackberry, WinMo and soon, iPhone or Android.
Laptops and cellphones (when not using GPS) can locate to a fair level of accuracy using geotagged Wi-Fi and cellular tower points in a database that Google's collected on its own, perhaps while doing Streetview photography. Or you can set your location manually. Google told me that there's no set standard for how often the map updates your location. Rather, they have an algorithm that depends on how often the device has moved, historically, and how much battery your device has left. You can also sign out of the service entirely, and set per user preferences on whether or not certain friends can see your location at all, or if only on city-levels of accuracy.
Google says its been useful for family members to find out if they're stuck in traffic, or on their way home. I tested the service with some people I know, but its been hard to say if its useful for a guy who has loved ones in generally predictable places. I generally know where my friends are, more or less, or can find out by texting them. I'd probably use this service more often while skiing or picking up friends at the airport, but not day to day. I mean, sure, I can turn off my privacy, but wouldn't people used to seeing your location at all times be suspicious if you suddenly turned off permissions when you want privacy?
Then again, maybe it would be nice to know when my father is playing golf in HK (all the time) or when Lisa is eating at her favorite place in Tokyo for Ramen, or where my brother is on tour with his band. That would be interesting, I suppose. But most of the time, most of us are in front of our computers. Until we're not. And that's where the phone clients come in.
Most phones will be able to keep the map location updated in the background. Except the iPhone. What the iPhone users can do, as a work around, is to lock the phone with the Google app running. That'll keep the phone updating until batteries die.
The Blackberry, WinMo and Symbian phones and laptops/Desktops can use Latitude now by downloading the most recent version of Google Maps or hitting Http://google.com/latitude. The iPhone gets it with an updated version of the increasingly powerful Google app, soon, as does the Android powered G1.