Smuxi is a cross-platform multi-protocol messaging program. While it was initially written for IRC, it now also supports Twitter, Facebook Chat, Google Talk, Jabber/XMPP, and Campfire as well. In addition to a traditional single-application mode, Smuxi also supports running a back-end engine separately from the front-end user interface. The front-end and back-end need not be run on the same computer. More technical users will often run the back-end on a small, always-on server (e.g., cloud hosting or a raspberry pi). In this client-server mode, message history and open tabs are synchronized across all connected front-ends, resulting in an experience similar to Slack. Smuxi's hook system allows users to write extensions in the scripting language of their choice. A number of community-supported extensions are available in the Smuxi-hooks GitHub repository. Smuxi is free software, distributed under the GNU General Public License, with source code available on GitHub. Smuxi is available for Windows, macOS, and Linux.