Gajim

Gajim is an instant messaging client for the XMPP protocol which uses the GTK toolkit. The name Gajim is a recursive acronym for Gajim's a jabber instant messenger. Gajim runs on Linux, BSD, macOS, and Microsoft Windows. Released under the GPL-3.0-only license, Gajim is free software. A 2009 round-up of similar software on Tom's Hardware found version 0.12.1 "the lightest and fastest jabber IM client".

Features
Gajim aims to be an easy to use and fully-featured XMPP client. Gajim uses GTK (PyGObject) as GUI library, which makes it cross-platform compatible. Some of its features: Gajim is available in Basque, Bulgarian, Chinese, Croatian, Czech, English, Esperanto, French, German, Italian, Norwegian (Bokmål), Polish, Russian, Spanish, Slovak, Swedish, Ukrainian and others.
 * Group chat support
 * Emojis, Avatars, File transfer
 * Systray icon, Spell checking
 * TLS, OpenPGP and end-to-end encryption support (OpenPGP not available under Windows until version 0.15),
 * Transport Registration support
 * Service Discovery including Nodes
 * Wikipedia, dictionary and search engine lookup
 * Multiple accounts support
 * D-Bus Capabilities
 * XML Console
 * Jingle voice and video support (using the "python-farstream" library, no support in Windows yet), FAQ says it's not supported yet, at least not under windows
 * OMEMO encryption
 * HTTP file upload

Third-party plugins
Gajim supports various third-party plugins (official list).