Plan B and Find My iPhone are free smartphone applications that attempt to locate a cellphone if its owner loses it. Once it detects the phone's location, the app relays the location to the owner.
Plan B is available to Android users, who can install it remotely using a Google Play account. After successfully installing it, the app sends the phone's location to the user's Gmail account. Find my iPhone, which works for Apple devices, comes pre-installed with iOS 5 or later. It also has the option of prompting the device to make a loud noise for two minutes, which is helpful if the phone is nearby.