Google today released Latitude for the iPhone, which allows other people to track your exact location in real time. If you're into sharing your every last move.
The one saving grace of Latitude is that you get to choose with whom you will share your exact movements. Beyond that, however, you have very little control over what information is shared. For example, you can't share just what neighborhood you're in, or share with a particular contact for a particular period of time. Also, Latitude does not require a proactive "check in" like Foursquare, so the app will continue to track and share your every move unless you get into the privacy menu and disable location sharing, which kind of defeats the whole purpose. But if you're really into being watched? Latitude might just be perfect.