When you roll up to your friend's house, you usually go searching through your contacts to call them. Pocket Doorbell saves you a step by determining your location and calling your nearest friend with the tap of a button.
Pocket Doorbell monitors your contact list for addresses, and when you open it up, determines your location using your phone's GPS. If you're at a contact's home, it'll put their name on the screen, and you can just tap the doorbell button to call them. If their address isn't in your address book, it'll ask you to register that location with a name in your contacts the first time you call, after which it'll remember that location for future visits. From then on, all you need to do is tap the doorbell to let your friend know you've arrived!
Pocket Doorbell is a $US0.99 download for the iPhone.
Pocket Doorbell [iTunes App Store]