Radio transmitter collars are great for energetic and outdoor dogs that love to roam. They come in several sizes with a corresponding receiver. Point the receiver in any direction; it will beep if your pet is nearby. (Think of the childhood game Marco, Polo.) Most devices can send and receive signals within 1 to 4 miles, depending on terrain and physical obstacles. Try Pettrax or Innotek collars, available at your local pet store.
Pros: Noninvasive; most radios include an accuracy guarantee and extended warranty.
Cons: Pet must be wearing the collar to be located.
Cost: from $130