Viber is a cross-platform IM and VoIP tool that lets you make free calls and send free messages to other Viber users on your smartphone and then carry on the conversation via chat on your PC. Viber syncs your contacts, messages, and history automatically between your phone and PC, so all you need to do is install Viber’s Windows app on your PC and the correct app for your phone (ours is Android), and let Viber do the rest.