If you have the latest version of Firefox, you already have Hello. There’s no account or sign-in required and nothing extra to download. Just start a conversation, send your friend a link and ask them to click it.

Google Chrome desktop browser and Chrome for Android include now support for WebRTC (Web Real Time Communications), an emerging web standard that helps  real-time audio and video capabilities to their web applications without the need for plug-ins or proprietary codecs.
There are various  apps has capabilities like voice calling, audio & video conferencing, video broadcasting, screencasting and screen sharing in the browser via simple JavaScript without requiring plugins.

Some of  web apps are like http://same.io,http://talky.io http://appear.in to Video /Voice chat Screen sharing without  installation or download anything. Only you require is the latest Chrome browser.

There’s a little setting in Chrome that you need to enable . Type chrome://flags in your Chrome’s address bar, find the setting that says “Enable screen capture support in getUserMedia()”, click the Enable button and restart your Chrome browser.