This one is naturally limited to iPhone users only (though the
general principle is identical for iPad and iPod Touch owners) and is
Apple’s proprietary method of locating a lost mobile. In order to use
the service you will need to have updated your iPhone to iOS 5 and
set up iCloud over at
iCloud.com.
Once
done you can login with your Apple ID credentials and click on “Find my
iPhone”. The website will then attempt to locate the device and provide
options for playing a sound, sending a message or even locking and
remotely wiping the contents of the device.