What IRC clients are available?

XChat

XChat is a graphical IRC Client with a GTK+ GUI. It has a look and feel similar to AmIRC for the Amiga. Special features include the mIRC extension DCC RESUME and mIRC color, multiple server/channel windows, dialog windows, and a plugin API.

It does pretty much everything you could want an IRC client to do.

XChat-Gnome

XChat-Gnome is very similar to XChat however it aims to have a more friendlier user interface and integrate better with your desktop. I find for starting out on IRC its much easier to get the hang off

sudo apt-get install xchat-gnome xchat-gnome-indicator

Screenshot:

IRSSI is the way to go: http://www.irssi.org/

It was even in the TV show 'Numbers'; check out the video on the Irssi homepage. (P.S. That was supposed to be funny)