In Debian based distros:

apt-get install jabber

Reading package lists... Done
Building dependency tree      
Reading state information... Done
The following packages were automatically installed and are no longer required:
  fakeroot dkms
Use 'apt-get autoremove' to remove them.
The following extra packages will be installed:
  jabber-common
The following NEW packages will be installed:
  jabber jabber-common
0 upgraded, 2 newly installed, 0 to remove and 185 not upgraded.
Need to get 175kB of archives.
After this operation, 655kB of additional disk space will be used.
Do you want to continue [Y/n]? y
Get:1 http://ca.archive.ubuntu.com intrepid/universe jabber-common 0.5 [4210B]
Get:2 http://ca.archive.ubuntu.com intrepid/universe jabber 1.4.3-3.4ubuntu1 [171kB]
Fetched 175kB in 1s (109kB/s)     
Selecting previously deselected package jabber-common.
(Reading database ... 113468 files and directories currently installed.)
Unpacking jabber-common (from .../jabber-common_0.5_all.deb) ...
Selecting previously deselected package jabber.
Unpacking jabber (from .../jabber_1.4.3-3.4ubuntu1_i386.deb) ...
Setting up jabber-common (0.5) ...

Setting up jabber (1.4.3-3.4ubuntu1) ...
Starting jabberd:

Here's a list of other XMPP servers