Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Facebook Messenger - Facebook Messenger is a faster way to message.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Digsby - Tagged makes it easy to meet and socialize with new people through games, shared interests, friend suggestions, browsing profiles, and much more.
Sipdroid - Free and open-source SIP/VoIP Client