Quassel IRC is a graphical, distributed, cross-platform IRC client, introduced in 2008. It is released under the GNU General Public License

GNU General Public License

The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

 for Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 and Unix-like

Unix-like

A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....

 operating systems, as well as Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 and Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

. Since the release of Kubuntu

Kubuntu

Kubuntu is an official derivative of the Ubuntu operating system using the KDE Plasma Desktop instead of the Unity graphical environment. It is part of the Ubuntu project and uses the same underlying system. It is possible to install both the KDE Plasma Desktop as well as the Unity desktop on...

 9.04 (Jaunty Jackalope) Quassel is Kubuntu's default IRC client. Quassel uses the class library Qt4.

Structure

Quassel is based on a Client–server model. The core application uses the Internet to connect to one or more clients, and also various IRC servers. The client does not communicate with the IRC server directly; it does so through the core. This way, the connection to the IRC network is maintained by the core, even though no clients are using it.
A "monolith" version of the application is also supported; which acts like a normal IRC client - there is no separation between core and client.

This system is similar to what Irssi

Irssi

Irssi is an IRC client program for Linux, Microsoft Windows, and Mac OS X. It was originally written by Timo Sirainen, and released under the terms of the GNU General Public License in January 1999.-Features:...

, WeeChat

WeeChat

WeeChat is a console IRC client, which is designed to be light and fast. It is released under the terms of the GNU General Public License 3 and has been developed since 2003. GTK+ and Qt interfaces are planned for the future....

 with GNU Screen

GNU Screen

GNU Screen is a software application that can be used to multiplex several virtual consoles, allowing a user to access multiple separate terminal sessions inside a single terminal window or remote terminal session...

, and Smuxi

Smuxi

Smuxi is a cross-platform IRC client for the GNOME desktop inspired by Irssi. It pioneered the concept of separating the frontend client from the backend engine which manages connections to IRC servers inside a single graphical application.-Architecture:...

 use.

Features

Quassel allows simultaneous connections to multiple IRC servers. Different identities can be created, and used on one or more of the servers the core connects to. These identities each contain default nicknames, fallback nicknames, away messages and so on. Each identity can be assigned to one or more servers.

Quassel stores discussion history in an SQLite

SQLite

SQLite is an ACID-compliant embedded relational database management system contained in a relatively small C programming library. The source code for SQLite is in the public domain and implements most of the SQL standard...

 database. When scrolling up through the chat window, older sections of chat are loaded automatically from stored logs. In this way, one can seamlessly view logs of past discussions.

Aliases, command "shortcuts", are also available; with these, a user can create an alias for a long command with many parameters.
The connection between the client and the core can be encrypted using SSL, and proxies are supported.

In a 2009 roundup of large IRC clients for Tom's Hardware, Adam Overa described Quassel as being "fully featured" with "tons of options," and, "even new users should have no problem connecting to servers and finding channels using the GUI tools for server presets and channel lists."