If you're anything like me, you're forgetful. You put your phone down somewhere, and just leave it there.
That's why there's Android Device Manager (Opens in a new page)! Download it ahead of time (Now!) from the Play Store and whenever you leave your phone somewhere, as long as you have wifi/data on, you can make your phone ring at the loudest possible volume! Great for finding your phone, or scaring your friends.
PS: It also lets you lock and wipe your phone remotely so theives can't take your information!
PSS: It's FREE!
***Edited to add labels***
I just want to add that you do not need to download this app on your phone. If it's an Android, go straight to the web page from your computer and you'll find your phone right there.
Having the app on your phone is only useful to locate another phone/device using the same Google account.