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
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 a group chat app in which messages are threaded by topic, helping you have long-running, organized conversations. Zulip's threading makes it easy for people not always on chat (like managers, remote team members, contractors, part-time workers, etc) to weigh in on...
The goal of Gajim is to provide a full featured and easy to use XMPP client. Gajim works nicely with GNOME, but does not require it to run. It is released under the GNU General Public License.
Empathy is a messaging program which supports text, voice, video chat, and file transfers over many different protocols. You can tell it about your accounts on all those services and do all your chatting within one application.Discontinued
Empathy is currently unmaintained. Last version, 3.25, released in August 2017, can be still downloaded from the official website.
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.
Fully open source instant messaging server and clients. No proprietary components or non-free dependencies. Backend in pure Go (GPL 3.0), with a command-line administration tool in python + gRPC. Clients for Android, iPhone, and Web - ReactJS (all licensed under Apache 2.0).
Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Google Talk, AIM, Jabber, ICQ, Facebook, MySpace and Twitter on ALL popular mobile platforms.
The widest selection of unique features that you won't find in any other IM client!.