The most commonly used app for finding a lost iPhone is Apple's own Find My iPhone app. This program is available from Apple's App Store for free and is compatible with iPhones running iOS 8.0, as of 2015.
Find My iPhone provides a way to track the location of an iPhone anywhere by using another iOS device where the application is installed. It also provides the ability to make a lost iPhone play a sound or display a message remotely, a useful feature for finding it under clothes or in dark spaces. Find My iPhone can put an iPhone into Lost Mode, a special mode that locks the iPhone's screen and displays contact information for the owner to make it easier for someone who finds the phone to return it or contact the owner. Lost Mode provides tracking history for the phone's location as well and can be used to remotely wipe data from the phone if it cannot be found or if it is suspected that the phone was stolen.
In addition to working with iPhones, Find My iPhone can also be used with iPads and Macintosh computers. However, older iPhones may not be able to use all of the features of this app, as it has been optimized for the iPhone 5 and subsequent models.