WeeChat is a free chat client, fast and light, designed for many operating systems. It is highly customizable and extensible with scripts. Features

Modular chat client: WeeChat has a lightweight core and optional plugins. All plugins (including IRC) are independent and can be unloaded. Multi-platform: WeeChat runs on GNU/Linux, *BSD, GNU/Hurd, Haiku, macOS and Windows (Bash/Ubuntu and Cygwin). Multi-protocols: WeeChat is designed to support multiple protocols by plugins, like IRC. Standards-compliant: the IRC plugin is compliant with RFCs 1459, 2810, 2811, 2812, and 2813. Small, fast, and very light: the core is and should stay as light and fast as possible. Customizable and extensible: there are a lot of options to customize WeeChat, and it is extensible with C plugins and scripts (Perl, Python, Ruby, Lua, Tcl, Scheme, JavaScript and PHP). Fully documented: there is comprehensive documentation, which is translated into several languages. Developed from scratch: WeeChat was built from scratch and is not based on any other client.