iHound is good for parents who want to put an insurance policy--of sorts--on the iPhone they bought for their kid, and also gives parents the ability to keep tabs on their child.
GadgetTrak is the tool for people whowant not only to track their iPhone, but to see who has it.
None of these services offered perfect iPhone tracking, and none of them helped me locate a lost device in all situations. And none of them will work if a thief decides to power off your iPhone. But all of them delivered plenty in the way of peace of mind. I like iHound's speed and its additional geolocation features, and I was impressed with GadgetTrak's photo-snapping prowess. Keep in mind that you don't have to choose just one: You could run all of these apps at once. But if I did have to choose, I'd go with Apple's Find My iPhone. Not only did it perform the best, acing most of my tests, it's free.