You can then contact them using text messaging, instant message or a phone call. The service is opt-in, so you won't be found if you don't want to be.
Initially, the system was available to Windows Mobile phones, most color Blackberry phones, Symbian S60 phones and Android phones with Google Maps 3.0 and above. Current G1 phones don't have version 3.0 yet, but it is coming in a system wide update.
How about the iPhone? Google says it is coming soon for both the iPhone and the iPod touch. There is no cost for the application, but people without unlimited data plans will see data charges. Use of the application will require a Google account. The service is available in 27 countries.
Google says it is sensitive about privacy, and Latitude users can, for example, let family members see exactly where they are, or let friends only know what city they are in. Alternatively, your location can be completely hidden from everyone (you sly dog).
Google is also offering a desktop version which allows you to input your location or determine it with wi-fi, and, of course, you can track your friends who are out and about.
The iPhone already has Loopt [App Store link] which has similar features, but Google is such a heavyweight in the web space that many people may sign up. Apple will have to update the Google Maps application on the iPhone to make it all work.