This is by far one of the most important apps for me. I've had friends recover missing phones with it, and I've also used it when I'm an idiot and leave my phone in my fiance's car and I have no idea where she's parked at the mall.
Here is another fun idea you can do with it. The best part of this program is that you can make it run only in the background when activated by SMS so whoever has the phone has absolutely no idea unless they happen to catch the "phone is doing something" icon at the right moment.