Meet the Peeple: a smart camera that lets you know when someone is knocking on your door or opening it. This smart camera mounts on your existing peephole and connects to your WiFi network to send you notifications when someone knocks on your door. The companion app runs on iOS/Android devices, allowing you to look through the peephole from anywhere.
Peeple lets you keep track all your knocks, so you can view who has visited you in the past. The device is easy to install or remove. You can find out more about it on Kickstarter.