The Brinno PHV133012 Digital PeepHole Viewer is a pretty cool device that displays who is at your door and captures the image without alerting people without alerting them. You could always enhance your gadget with these two add-ons:
Brinno KNS100 Knocking Sensor: takes pictures of anyone knocking at your door by sensing the vibration produced by knocking.
Brinno MAS100 Wireless Motion Sensor: mounts outside your door and triggers the digital PeepHole viewer to take pictures of anyone at your door. You can capture photos of people even when you are not home.
There are other security cameras that can capture the picture of people at your door. If you already have a Brinno, these add-ons are worth checking out.