Presselite's Phone Tracker is a joke — but don't take my word for it. Look it up in the App Store, and read Presselite's explanation. Phone Tracker's main purpose — check that, its only purpose — is to display your iPhone's GPS location and pass it off as somebody else's.
So let's say you are in a bar. Because there is nothing inherently interesting about you or your life, you decide to impress a new acquaintance with your ability to track his or her phone. You enter the phone number into Phone Tracker, and voila, it displays your - and thus, your acquaintance's - current location.
Here's where the joke wears thin: if you are able to get an acquaintance's phone number, you really don't need to impress him or her with Phone Tracker. If you are not, it is unlikely you will be able to acquire said digits under the pretense of being capable of stalking him or her wherever they go.
There would be one upside to the joke app: if said acquaintance is stupid or drunk enough that he or she gives you said digits because of this app, he or she is an utter idiot. Having established his or her horrible judgment, it is not a leap to conclude that other lapses of judgment are within reach.
Here's the kicker—it's not even a good joke, at least not all the time. I tried it out using a friend's number, to demonstrate the principal. The radar animation started spinning. The map started zooming in. And lo and behold, my friend's phone was pinpointed—in the next town over.
Oh. My. Freaking. Gawd. Granted, the fault may be with Apple's implementation of GPS. But still, this app does one thing, of very little value, and it cannot do that one thing reliably. Oh, and if you're wondering, Phone Tracker does not remember the digits you've just scored.
I know the dollar isn't what it used to be, but it's more than this.
By the way, if you think Phone Tracker is actually an app meant to do something of any value whatsoever—say, to help you find your lost phone—you'll be disappointed. You have to have your phone to locate your phone, perhaps a mile or two from its actual location.
1/10, because I'm in a generous mood.
Hint: If you're concerned that someday you might need a reliable way to track down a lost iPhone, sign up for MobileMe. Apple's cloud service includes Find My iPhone, a service that is designed to help panic-stricken iPhone owners locate their lost device.