Conversations.im - XMPP client designed with ease of use and security in mind
tSIP - tomeko.net/software: SIP client for Windows.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Telephone - Telephone is a VoIP program which allows you to make phone calls over the internet.
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.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.