Find My iPhone (for iPhone 4, iPad or 4th Generation iPod touch)

This is a must have app for your iOS device.

If you lose your iPhone, iPad, or iPod touch while on the go, simply install the Find My iPhone app on another iOS device, launch it, and sign in with your Apple ID (Find My iPhone must already be configured on the missing device). 
If you lose your iPhone, iPad, or iPod touch while on the go, simply install the Find My iPhone app on another iOS device, launch it, and sign in with your Apple ID (Find My iPhone must already be configured on the missing device). 

Download link: http://itunes.apple.com/us/app/find-my-iphone/id376101648?mt=8#