Find My iPhone is one of those apps you don't need until you need it. When you need it, it's priceless (even though it is free). Find My iPhone works on iOS devices and requires an iCloud account and a valid Apple ID. When turned on, Find My iPhone sends the geolocation data of your device to the servers at regular intervals, allowing you to find your phone from any other device you can log into (including Web browsers). Find My iPhone is installed by default with iOS 5 and 6, but must be turned on to be active.
Find My iPhone can be used to locate a device. Simply start the Find My iPhone app and enter the Apple ID login and password, then the screen shows a map with the last reported location of the devices attached to that account displayed. You can actually watch the device in near real-time, so it's a great tracking app. Even more important, if a device is lost or stolen, as long as it can be found you can lock the device remotely using a four-digit code as well as display a phone number on the device's screen, or you can initiate a wipe of the device if you want to protect its contents. We've used Find My iPhone in real lost phone incidents, logging on from another iDevice and locking the lost iPhone.
Find My iPhone works, as long as the device being tracked reports its position and can receive commands. Fortunately, this means essentially in all cases as long as the device is turned on and not wiped first, you can locate it. Of course, you need to be able to access your iCloud account from another device, but as long as Find My iPhone is turned on this is a great free app from Apple.