Free app, $199 (£121) for device
SkyBell is a very neatly packaged doorbell with a built-in camera and microphone. It connects to your home Wi-Fi, and the free smartphone app allows you to see and talk to anyone at your door. Even if you're not at home. It also has a motion detector, so they don't necessarily have to press the button to activate the SkyBell. The device itself is a little pricey, particularly if you want it shipped outside of the US (add $19.95), but the concept is rock-solid.