letsBrik - A Skype Alternative that is entirely web browser based.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
Amivox - Would you like to make international calls either free or at local rates ?
BetterChat for WhatsApp - Gorgeous new full-featured WhatsApp client for the Mac.
KeepCalling - Best Rates for International Calls. No Contract, Expiry, or PIN. Use Any Phone, Smartphone or PC.
Number To Whatsapp - A utility tool which lets your start a Whatsapp conversation without saving the contact of the said person first.