Google Latitude allows you to tell your location to your friends via cellphone

Google Latitude allows you to broadcast your location to your friends, family, and colleagues. If you have the GPS feature enabled in Google Maps on your mobile phone, you can keep track of where everyone is at any given time, and from within the app you can call, email, or IM them. If you don’t feel like being tracked, Just hide yourself from select “friends,” or manually enter a false location. You can control what your location is and who gets to see it. More about Google Latitude and start using it.