AgiliaLinux:

Just mpkg-install gajim

Archlinux:

Just pacman -S gajim

Debian GNU/Linux:

Just sudo apt-get install gajim
A daily package from git is also built. To use it:
Remove Gajim:
sudo apt-get remove gajim
Add the GPG key:
wget -q https://gajim.org/gajim.key -O - | apt-key add -
Add the repository:
sudo sh -c "echo deb ftp://ftp.gajim.org/debian unstable main > /etc/apt/sources.list.d/gajim.list"
Update the repository:
sudo apt-get update
And install Gajim by doing:
sudo apt-get install gajim-nightly

Fedora:

Just dnf install gajim

Flatpak:

You can find Gajim on flathub.
Instructions to build flatpak package are available here

FreeBSD:

Gajim is available in FreeBSD via FreeBSD ports. Just cd /usr/ports/net/gajim/ && make install clean. Find out more.

Gentoo:

Just emerge gajim

Mandriva:

Just urpmi gajim

openSUSE:

One click install for openSuSE 12.2, 12.3
For other versions, get it here

PLD:

Just poldek -U gajim

Slackware:

You can get the Slackware package of Gajim here.

SourceMage:

Just cast gajim

Ubuntu:

Just sudo apt-get install gajim

A daily package from git is also built. To use it:
Remove Gajim:
sudo apt-get remove gajim
Add the GPG key:
wget -q https://gajim.org/gajim.key -O - | apt-key add -
Add the repository:
sudo sh -c "echo deb ftp://ftp.gajim.org/debian unstable main > /etc/apt/sources.list.d/gajim.list"
Update the repository:
sudo apt-get update
And install Gajim by doing:
sudo apt-get install gajim-nightly

Windows:

There is a 32 bits Gajim installer and a 64 bits Gajim installer
There is also a 32 bits Gajim portable installer. and a 64 bits Gajim portable installer.

ZenWalk:

Just netpkg gajim

Other Linux / source:

The tarball version of Gajim is here. For information on how to build Gajim, see the README (in English).
If you want to use the last unstable development version, then you should checkout the git repository.

git

The git repository is available at this address: https://dev.gajim.org/gajim/gajim and viewable here.

Under GNU/Linux

In order to download Gajim sources, here is what you have to do: You first have to install git (apt-get install git under Debian)
Then in a console, enter the following commands:

git clone https://dev.gajim.org/gajim/gajim.git gajim

Install dependencies, and build Gajim. Please see the README (English) for details.

You can run Gajim by typing the following:

cd gajim and then ./launch.sh

Once you have the repository, you can update it periodically so it matches the latest version from the main repository with the following command:

git pull

Under Windows

You first have to install git for windows. In order to do that, download the binary here
Once git is installed, the commands are the same with those under GNU/Linux. Only difference is instead of launch.sh you run launch.bat. Enter them in an MS-DOS command line and you're done.
To run Gajim, you need external libraries. You can find a tutorial on the way to install them On the Wiki.

Daily Snapshots

Everyday on 01:00 (GMT+1 time) gz tarballs are produced from git. You can use them if Git is broken for you at the moment. Here to get them.