Tagg is at CES 2012 showing off their tracking app for pets. The hardware receiver is extremely lightweight and attaches directly to your pet's collar. Once activated, the receiver will continuously monitor your pet's location. If they happen to wander outside your home area, you will receive a text or e-mail notification. From there you can track your pet's location via the free application or iPhone and iPad.
If you want to take your pet for a walk or travel outside your home area, a button press on their collar will deactivate notifications but continue to GPS track. Tap it back on when you'd like notifications to be active again.
The Tagg collar is currently available for $99.95. The service will run you $7.95/month with your first month free.