Adium

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iMessage

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.

Dialog Messenger is a corporate messenger for teamwork and secure communication.

Facebook Messenger is a free standalone chat application that is well integrated with the social network, Facebook. With Facebook Messenger you can stay in contact with friends and family, and other contacts from Facebook. The applicaiton has a ton of built-in features that make communication fun and engaging. Send images, gifs, video messages, voice notes, and stickers or share content from around the web. Facebook Messenger automatically syncs with your Facebook contacts without the need of adding phone numbers or identification manually.

WhatsApp is free messenger app for chat and voice over IP, owned and managed by Facebook. The application allow users to send text and voice messages, make voice and video calls and share images, media files and user locations with friends and family.