Conversations.im - XMPP client designed with ease of use and security in mind
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
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.
YakYak - Desktop client for Google Hangouts