Viber is a fastly-growing VoIP application that supports free text messaging, free calls, and other social services. Viber is free to use and doesn't require any registration process. Viber has been available for Windows, OS X, iOS, Android, Windows Phone, and even BlackBerry and Nokia platforms, while with the 4.0 release yesterday it officially supports Linux.
Back in August I wrote about the Viber Linux client originally appearing while now it's been officially released as part of version 4.0.
Those interested in fetching Viber 4.0 for Linux can stop by Viber.com.