From Viber Media:

Viber for Windows lets you send free messages and make free calls to other Viber users, on any device and network, in any country.

Viber syncs your contacts, messages and call history with your mobile device, so you can start the conversation on the Viber mobile application and continue from the comfort of your PC.Best-quality HD voice callsVideo callsFree text and photo messagesGroup conversationsNo registration, passwords or invitations requiredContacts and messages sync between your mobile and WindowsTransfer ongoing calls between devices

Download Viber, enter your phone number, and start connecting with your friends.