Google Latitude is service that tracks your location using either your GPS-enabled phone or the IP address of the computer you are using. I have my phone set to report my location automatically on a daily basis. Google publishes this information publicly on a city-sized basis (e.g. Google will tell you that I am in Seattle, but not the street address), which you can see on the right side of this screen. Friends and trusted contacts can get much more specific information on my location when they log in to Google Latitude.
Publishing your real-time location certainly isn't for everyone. The only reasons I'm making this information public is because A) I live with a roommate and we're rarely out of town at the same time, and B) I don't really have anything worth stealing at my apartment anyway.