Google Device Manager is built into all android phones and is activated by default.
Log into Google Device Manager using your gmail login to find your phone on a map, ring the phone, lock the phone, or wipe the phone.
Since the default setting is to track the phone location, this will work for a lost phone even if nothing has been installed.
There are many apps that may be installed on android phones to do this, but Google Device Manager has the advantage that it is already installed and activated.One free app is Lookout. An advantage this app has is that it can be configured to send an email containing the phone location whenever the battery level gets down to 10%.
You may want to bookmark this link to give to anyone who has a lost phone and asks you if there is any way to find it.