Marco Pollo is a $1 iPhone app that runs in the background waiting for you to lose your phone and yell 'Marco'. It then responds 'Polo'. You can also program it to respond to a different name, but it will always respond with Polo. BOOTYJUICE. "Polo." There you are!
I tried Marco Polo around the house, first putting the phone on a dresser. That was an easy one -- I said "Marco" and the phone replied. I also put the iPhone under a couple of blankets, and that worked too. Then I put my iPhone in a drawer in the kitchen, and while the sound was muffled, I could hear it respond. Obviously, if I got too far away -- like two rooms away -- it wouldn't trigger. But I would be quite happy to shout out a couple of different "Marco's" to find my lost phone.
I don't have the tools for precise measurement, but I didn't see any significant drain on my battery while the app was running. My only suggestion for the developers is that once the phone says "Polo", it might be nice if it emitted a repeating tone to make it easier to pinpoint without having to go through the house yelling "Marco" over and over.
Call me an antique, but I still find my lost phone the old fashioned way: tearing the house apart. Then I use somebody else's phone to call and listen for the buzzing (I always keep it on silent). If that doesn't work I accuse my roommates of stealing it, before laying in the shower and trying as hard as I can to remember if I KNOW I had it when I got home from the bar last night. That is never easy. Eventually, I get so angry I catch myself yelling at my penis for no reason. LOOK AT YOU -- YOU'RE LIKE AN EARTHWORM DRYING UP ON THE SIDEWALK.
Thanks to Thaylor H, who agrees the easiest way to find your phone is only having a corded land line. I miss those days.