The WhereAmI Project is an attempt to use GPS tracking to find your friends and have them find you. The project, an experimental beta at the moment, has its only users in WhereAmI's creator, Nagi Babu Punyamurthula, and his friends. WhereAmI works by letting a user transmit their GPS location to a central server using an application that runs on a GPS-enabled mobile device or a similarly-equipped laptop with an internet connection. On the server, a web service runs to collect the GPS data send from the mobile device. The third part is an ASP.NET AJAX-enabled web page running a Microsoft Virtual Earth Mapping Control to plot the user's position on the map. Users running the WhereAmI app can not only find each other, but also things around them like ATMs, shops, and bars. Of course, there is a big disable button for the times you don't want to be found.
If you're would like to try this project out for yourself, you can contact Nagi to get a Profile ID. Once you have that, you can download either the laptop version or the Pocket PC Phone version of the application and install it. You'll configure the application with the Profile ID provided to you. And, as long as you have a GPS receiver connectivity to either of these devices (either built-in, Bluetooth, or direct cabled), you'll be all set.