Adium

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Discord

Jitsi is an open-source project and software toolset that allows you to easily build and deploy secure video conferencing solutions. At the heart of Jitsi are Jitsi Videobridge and Jitsi Meet, which let you have conferences on the internet, while other projects in the community enable other features such as audio, dial-in, recording, and simulcasting.  Jitsi supports popular protocols such as SIP, XMPP/Jabber, ICQ/AIM, Windows Live, Yahoo!, Google Hangouts, OTR, ZRTP and more.

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Pidgin is a chat application which lets you log into accounts on multiple chat networks simultaneously. Pidgin is developed and released fully free and open-source and provides a plugin architecture that allow anyone to build extensions and connections to new networks and third-party services.

Mumble is a free and open-source voice over IP application primarily designed for gamers. The application features a customizable in game overlay, application skins, conversation recording, and advanced voice activity detection. Anyone can download and use Mumble for free, but if you want your own server, you can rent a managed solution from the company behind Mumble, or host your own.

TeamSpeak is a VOIP application that provides military-grade encryption making it ideal choice for your company's, teams and gamers internal, closed-loop communications systems. TeamSpeak is known for its lag-free performance & unparalleled reliability and uptime.

Zoom is a commercial video and web conferencing software that can be used for webinars or team collaboration and communication. The software is developed by the Chinese and American company Zoom Video Communications, Inc. Zoom provides a cloud platform that is used and loved by many enterprises and fortune 500 companies, due to the easy-of-use and infrastructure-less nature of the platform.