Apple's "Find my iPhone" service offers free tracking for a lost or stolen iPhone. As long as the location-based service is activated through the Settings menu, the owner can log into the connected iCloud account to track, lock and even wipe all personal data from a lost phone.
Find my iPhone uses the phone's GPS receiver to determine its current location. When set to "lost mode", it reports that information to iCloud any time it is able to connect to the Internet via Wi-Fi or cellular service. The owner can log into iCloud and view a map of the phone's most recent connection. Owners can even receive driving directions by clicking the car icon in the top right corner of the map.
Once a phone is in lost mode, the owner can also log into the device to play sounds or post messages on the screen, identifying the phone as lost and giving information to the finder. In the case of a stolen iPhone, the owner can also lock the device and prevent it from being used or reactivated without a passcode. In extreme cases, Find my iPhone can be used to wipe a stolen device to prevent personal data from falling into the hands of a third party.