Using the simple 'OK Google' voice command, you can now send messages through Whatsapp, Viber, WeChat, and a variety of other apps. Sometimes, you just don't want to pick up the phone and type out a reply, and now you don't have to. You have been able to send Hangouts messages using your voice already, so the expansion is great to see.
Getting started is easy, all you have to do is say "Ok Google, send a Whatsapp message," and then it will ask who you want to send it to, and then the message. The new functionality is only available in English, but this time it is not limited to the US. Google is working to bring this to more languages in the future.