Google Duo is a new video call app that was announced during Google I/O 2016 but finally launched three months later in August. Available for Android and iOS, Duo aims to be a simple and amazing video calling app that, along with partner Allo, will take over for Google Hangouts. Duo is extremely simple for person-to-person video calling, keeping things simple to let you focus on the person instead of the process of the call.
A key feature called "Knock Knock" lets you see a glimpse of the person when they call you before they can see you, so you get a feel for what the situation is and whether a call is appropriate to pick up at the time. This also means the connections are seemingly instant: as soon as you swipe to answer, your call is started.
Duo also uses QUIC for fast connections and was built by the team behind WebRTC. The app will also switches easily between Wi-Fi and cellular connections, has settings to cut back on cellular data use, and is designed to gracefully degrade video quality to keep the conversation up to speed and smooth.