Log on to globalpetfinder.com to build virtual fences where your pet can wander. Your customized boundaries are automatically downloaded to the tracking device, so if your pet goes outside the designated area, you will be alerted via cell phone, PDA, or computer. You can also dial the word "found" from your phone to get your animal's current location.
Pros: GPS allows for 24/7 monitoring and can alert you if your pet is in dangerously hot or cold temperatures.
Cons: Not all Internet service providers offer coverage (call yours to check). Requires a 2-way wireless device like a cell phone, PDA, or Internet, as well as a 1-year minimum subscription.
Cost: $290 gets you Global PetFinder service, a snap guard that attaches the device to your pet's collar, batteries, and a charger. There's also a one-time activation fee of $34.99; monthly plans from $18.