I've been playing with Google's new product - Latitude. This allows you to publish your location from either a mobile phone (in my case WinMo 6.1 on an HTC Diamond) or from your computer - either automatically running Gears or manually.
It is pretty easy to set up although you might have to specifically point your browser to http://www.google.com/ig?hl=en to stop the 'not supported yet' message that the V1 interface shows.
It promises to display the location of all your friends and track them as they move about. Of course, you can turn off reporting if you are on a private trip somewhere and to track people you must be invited by them. Seems pretty good to me!