The DoorBot is a doorbell with a difference. It is equipped with a small camera, a microphone and a speaker, and connects wirelessly to your mobile device. When somebody rings your doorbell, an app on your iOS or Android device notifies you, lets you see who is there, and allows you to speak to them.
You can wire DoorBot to an existing doorbell so that it will ping your smartphone and ring your actual doorbell at the same time; it gets power from the doorbell wiring. Otherwise, just hang it and let it run it off its internal battery.
For an additional $150, you can pair your DoorBot with Lockitron, which lets you wirelessly unlock your door as well.