free, distributed and user-friendly IRC / Twitter / XMPP / Campfire / JabbR client for Linux, Windows and Mac OS X based on GNOME / GTK+

Ubuntu users can install Smuxi from this PPA, just run the following commands:

sudo add-apt-repository ppa:meebey/smuxi-stable
sudo apt-get update
sudo apt-get install smuxi

Now you can start Smuxi from the Unity Dash.

Screenshot of Smuxi on Ubuntu