It's technically possible to identify someone's location James Bond-style by triangulating the cell towers that person is using. Surreptitiously tracking someone through a cellphone, however, is not allowed. The best solution is to either subscribe to your carrier's paid family tracking service, or use Google's free Latitude service. Both require consent from yourself and whomever you wish to track. Google does not charge for use of this system.
How it Works
Google's Latitude product uses the global positioning system (GPS) feature in newer cellphones and allows you to see your friend's location and status messages and share yours with him. Location sharing starts when you and a friend agree. You control the privacy level. You can set, share or hide your location. You can set a city-level location too for a bit more privacy.
Sign up for Google Latitude at http://www.google.com/latitude. The sign-up process will offer you the opportunity to invite one or more friends. They will receive an invitation. The process, in due course, will send an SMS message to you and your friends' phones. The link will instruct you on how to download the Google Maps application to your phone. You will get the best results with a larger-screen smart phone device. Or, type this URL into your phone's browser: google.com/latitude
Getting Going with Google
The Google Maps application on your phone has a Latitude option in its menu. This tab allows you to add friends (Invite them) and set your privacy settings. From the Google Latitude privacy menu, you can choose to detect and share your location automatically; set your location manually; and hide your location from all friends. The Google Maps application uses your phone's data connection to receive and transmit the GPS position of the phone. The accuracy will be determined by the accuracy of the method used by the phone to track your position, such as GPS or triangulation. GPS is more accurate.
Simply keep the Google Maps application in memory on your phone, and as you are moving around, the phone will broadcast its position via the phone's data connection to the Google servers. Google will map your position and send that position to your friend's map.