There are a number of free apps that can be downloaded to a cellphone, such as Prey, Find My iPhone or Plan B, all of which can track a device remotely once installed. Although the options are more limited, it is also possible to track older GSM phones as well to some extent.
Prey is open source and free, and also works on both Android and Apple devices, making it a good all round choice. To use, simply download the app, and follow the onscreen instructions. Once the software is installed on the device, it will run in the background at all times, enabling easy location.
To locate a phone after it has been lost, Plan B or Find My iPhone are the best choices. Plan B can be downloaded from the Play Store to any device. The user can then log in to their Google account, and install the app on the missing device. Once complete, Plan B will email the device's location to the user's email account.
Find My iPhone operates in a similar way. The software comes installed on most Apple devices already, so locating a lost cellphone is simply a case of logging in with an Apple ID. The app will then display the location of the device.