openSUSE 11.3 users can install Smuxi by using this 1-Click install:

Debian 6.0 (Stable / Squeeze)
Debian 6.0 users can install Smuxi from the backports.debian.org repository, just add the
following line to your /etc/apt/source.list file:
deb http://backports.debian.org/debian-backports squeeze-backports main
After that run:
apt-get update
apt-get -t squeeze-backports install smuxi
Now you can start Smuxi from the GNOME or KDE menu.
Debian 5.0 (Old Stable / Lenny)
Debian 5.0 users can install Smuxi from the backports.debian.org repository, just add the
following line to your /etc/apt/source.list file:
deb http://backports.debian.org/debian-backports lenny-backports main
After that run:
apt-get update
apt-get -t lenny-backports install smuxi
Now you can start Smuxi from the GNOME or KDE menu.
Debian Testing and Unstable
Debian/Testing and Debian/Unstable users can install Smuxi from the official Debian repository, just run the following command:
apt-get install smuxi
Now you can start Smuxi from the GNOME or KDE menu.
Unofficial Debian Repositories
In case the official repositories from above are not providing the latest Smuxi version yet, you can use these instead:
For Debian/Testing and Debian/Unstable add the following line to your /etc/apt/source.list file:
deb http://debian.meebey.net/sid/smuxi /
For Debian 6.0 add the following line to your /etc/apt/source.list file:
deb http://debian.meebey.net/squeeze-backports/smuxi /
For Debian 5.0 add the following line to your /etc/apt/source.list file:
deb http://debian.meebey.net/lenny-backports/mono /
deb http://debian.meebey.net/lenny-backports/smuxi /
After that run:
apt-get update
apt-get install smuxi
Now you can start Smuxi from the GNOME or KDE menu.
NOTE: Ubuntu/Lucid (10.4) contains Smuxi 0.7.1, but it's recommended to use 0.8 instead!
Ubuntu/Lucid (10.04), Ubuntu/Oneiric (11.10) and Ubuntu/Precise (12.04) users can install Smuxi from this PPA, just run the
following commands:
sudo add-apt-repository ppa:meebey/ppa
sudo apt-get update
sudo apt-get install smuxi
Now you can start Smuxi from the GNOME or KDE menu.
Gentoo users that are using ~amd64 or ~x86 can install Smuxi by running:
emerge smuxi
Now you can start Smuxi from the GNOME or KDE menu.
Smuxi is in AUR, so just download the PKGBUILD and run makepkg. Alternatively if you're using yaourt just run the following command:
sudo yaourt -S smuxi
Now you can start Smuxi from the GNOME or KDE menu.
Smuxi is in Conary, so just run the following comand to install it:
sudo conary update smuxi
Now you can start Smuxi from the GNOME, KDE or XFCE menu.
Download: smuxi-0.8-1-bin.tar.gz
This package contains pre-compiled Smuxi binaries plus the required
libraries (log4net, Nini, SmartIrc4net), suitable for Linux (using
Mono) users.
Linux users using the binary tarball have to make sure the required runtime and libraries are installed in order to run Smuxi correctly!
You need the Mono runtime (>= 1.2.6) and the GTK# libraries (>= 2.8) which you can download here:
http://www.go-mono.com/mono-downloads/download.html
Alternatively you can install Mono and GTK# from your package repository of your Linux Distribution, just make sure they are the right version!
After you have installed those you can run the smuxi-frontend-gnome script from the binary tarball or run the following from a terminal:
mono smuxi-frontend-gnome.exe
Smuxi is in the official FreeBSD ports tree, so just run the following comand to install it:
make -C /usr/ports/irc/smuxi/ install
Now you can start Smuxi from the GNOME or KDE menu.

Download: smuxi-0.8.11-3-dev-osx.zip
Mac OS X users can download this experimental package. You have to download and
install first the Mono runtime though:

This tarball contains only the source code of Smuxi and is intended for package maintainers of Unix-like operating systems as it requires you to compile Smuxi and the needed components yourself.