The Google dialer is what ships on all Nexus phones, but OEMs usually create their own vastly different dialer app. Beginning with the v2.05 update, you can install Google's version on most other phones too. This update rolled out a few weeks ago, but it seems that people are still just noticing, so let's get this all out in the open.
Viber is what you get when you take a free VoIP service, create an app for it, and smother it with purple. Any two people with the app installed can communicate with one another via voice or text entirely for free. Now the team is adding in the ability to make low-cost calls to any dedicated phone number.
Users can buy credits for "Viber Out" via in-app purchases within either the mobile or desktop software.
The transition from Google Talk to Hangouts added a lot of new features, but making phone calls from Gmail didn't come along for the ride. There was still a roundabout way of adding phone numbers to a Hangout, but it was too awkward to be useful. Finally, Google has announced that phone calls are again possible from Gmail, but that's not all. Phone calls are now built into Hangouts in Gmail, Google+, and the Chrome Extension.
Out of all the things that we use smartphones for these days, calls come in pretty close to last. We're too busy texting, taking pictures, surfing the web, getting directions... you get the idea. What if we could incorporate all of these things into our phone calls though? That's exactly what Thrutu aims to do, and the newest update to the app makes most of that possible. Here's a video to get a better idea:
Every feature isn't highlighted in the video, though - it can actually do even more than that.