Jitsi Desktop (previously SIP Communicator) is a legacy audio/video and chat communicator with full encryption, precedessor of Jitsi Meet .
Jitsi Desktop is not actively developed anymore: last version, released in October 2017, can be still downloaded from GitHub. Its successor is Jitsi Meet
Skype is the most popular free voice-over IP and instant messaging service globally. It allows users to text, video and voice call over the internet. Users can also call landlines and mobiles at competitive rates using Skype credit, premium accounts and subscriptions.
WhatsApp is an iPhone and Android messaging app which allows you to exchange messages without having to pay for SMS.
Wire is the most secure collaboration platform featuring messenger, voice, video, conference calls, file-sharing, and external collaboration –all protected by the most secure end-to-end-encryption.
Zulip is the only modern team chat app that is designed for both live and asynchronous conversations. Zulip topics create a separate space for each discussion, so different conversations will never get in each other's way.
With Viber, everyone in the world can connect. Freely. More than 460 million Viber users text, make HD-quality phone and video calls, and send photo and video messages worldwide over WiFi or 3G - for free.
Video conversations with up to 8 people in your browser - no downloads, no login. Just name your channel and you are on. Works in all modern browsers, even mobile.
LINE is a Japanese proprietary application for instant messaging on smartphones and PCs. Line users exchange text messages, graphics, video and audio media, make free VoIP calls, and hold free audio or video conferences.
Line2 is a VoIP phone service that lets you call & text from a separate phone number via WiFi or 3G/4G/LTE. At a small premium, Text & call ANYONE in the US/Canada, forwarding & more. Pick any phone number.