The PetRescuers iPhone app sounds pretty basic—It's got a way to enter information about lost pets and a search to identify a pet you may have found—but it has the wonderful potential to reunite many pets and owners.
Basically, if you happen upon a lonely looking pet you could check the app to see if it happens to be missing or alternatively, you could use the app to alert others of your lost pet. Simple, but potentially very effective if you live in a community full of iPhone users.
The app is free and available in iTunes now. iTunes]