Since 2008, one of the handy features of Apple's MobileMe cloud service has been Find My iPhone.
Once an iPhone was set up to work with it, a MobileMe user could use the service to locate a lost or stolen iPhone, display its whereabouts on a map, send it a message, or, if need be, wipe it of data.
Accompanying today's upgrades to the MobileMe service, Find My iPhone has also been spun out into a free iPhone/iPad app, enabling you to find one lost device from another one.
That's handy, since you're probably running around looking for your lost iPad anyway, so you can track it in real time.
The only catch is that you still need a MobileMe account to use the app.