Telegram Purple is a Pidgin (and other Libpurple-based messengers) plugin which adds support for Telegram.

Viber is an application that sits somewhere between Skype and WhatsApp, which can be used to make free calls, send text messages, photos and video messages without having to register (all phone contacts that have installed Viber are listed as Viber contacts).

Initially, Viber was a mobile-only application but a few months ago, a desktop client has been released for OS X and Windows. Earlier this month, Viber Media Inc. has released Viber for Linux as a public beta. For now, there are only 64bit binaries available for download.

FBMessenger is a stand-alone Facebook Messenger app for Linux, very similar to the official Messenger for Windows.

This isn't an official Facebook application but it has been created by a Facebook developer and comes with the following features:

SkypeTab-NG (NG stands for Next Generation) is a tool that adds tabs to Skype for Linux. Besides providing a tabbed chat window, the SkypeTab-NG also embeds both the conversation and the main Skype windows in a single window.

SkypeTab is not new, but it was recently rewritten, fixing most bugs from the previous SkypeTab, like the annoying laggy window movement bug. Also, SkypeTab used to only work with Compiz, Openbox and Xfwm but the new version should work with other window managers too, like Kwin, etc (I've actually tested it under Kubuntu 11.10 Oneiric Ocelot and it worked just fine).

Google finally enabled voice and video chat for the Linux users. To use it in Ubuntu, go to this page: http://www.google.com/chat/video click on "Install" and download the deb and install it.

Voice and video chat for Linux currently supports Ubuntu and other Debian-based Linux distributions, and RPM support will be coming soon.


Update: for non-Debian based Linux distributions, try this (thanks to rmull @ reddit):

32bit:

mkdir goog; cd goog
wget http://dl.google.com/linux/direct/google-talkplugin_current_i386.deb
ar vx google-talkplugin_current_i386.deb
sudo tar xvzf data.tar.gz -C /

64bit:

mkdir goog; cd goog
wget http://dl.google.com/linux/direct/google-talkplugin_current_amd64.deb
ar vx google-talkplugin_current_amd64.deb
sudo tar xvzf data.tar.gz -C /

Then restart Firefox.

Skype has just been uploaded to the "partner" official Ubuntu 10.04 Lucid Lynx repository - correct me if I'm wrong but I think this is the first time Skype is uploaded to an official Ubuntu repository (other than Medibuntu obviously - which by the way doesn't include Skype anymore).