Loopt is a relatively new service that provides a tool for people who just have to tell others where they are.
Whether it be for finding friends, or seeing how long it will be until they arrive for an event, the service combines cell phone technology with knowledge of where the phone is, to broadcast this information to your select group of friends.
Perspective: I think Loopt has a place, and judging from their user base of over 100,000, it seems to have found a niche. I suspect they will need to link this service to the major Social networks like Facebook, and Myspace to reach a critical mass. This model also presents interesting localised advertising opportunities such as restaurants, bars, and even targeted sales items that may be matched to a user profile.