This week, Google released their answer to Apple's FaceTime: Duo. A 1-to-1 video calling app that works on both Android and iOS devices. It uses the contacts list in your phone, so you don't have to sync or create accounts; The calls are encrypted on both ends for security; It automatically adjusts the video quality to compensate for changing signal strength without dropping the call, and it switches between 4G and WiFi during the call.
And here's the coolest part: it has a feature called “knock knock” which lets you answer the call before you actually answer. It lets you see the image from the caller's camera before you answer the call so you can see what kind of mood they're in.