Quassel is a cross-platform, graphical Internet Relay Chat (IRC) client. Users can opt to run the core of the application as a standalone server on an always-on machine, which they then connect to with the Quassel client. In this client/server mode, Quassel functions similarly to Irssi or WeeChat inside GNU Screen or tmux; users maintain a persistent connection to their IRC networks that they can splice into from any of their computers. Quassel can also run in a monolithic mode, where the client and server are all wrapped up in a single application. In this mode, Quassel is similar to more traditional chat programs. Quassel executables can be downloaded from the website for Windows, macOS, and Linux. Many free operating systems also include Quassel in their package repositories. Quassel is free software, licensed under the GNU General Public License, with source code available on GitHub.