Internet Relay Chat (IRC), a form of Internet chat was created by Jarkko Oikarinen in 1988. Here is a list of Linux clients that will get you connected to IRC Networks. For an extensive list of IRC clients and their features, see the Wikipedia Comparison of Internet Relay Chat clients page.
Irssi is the self proclaimed client of the future and my favorite IRC client. Irssi is a terminal based, feature-rich, highly configurable, themed environment with support for Perl scripts. The project website hosts a large variety of themes and scripts that can be used with Irssi.
BitchX is a feature-rich and highly configurable, terminal based IRC client based on ircII. BitchX supports custom scripts, which can be written in TCL.
Konversation is a graphical IRC client for the KDE that supports multiple identities and theme support for nick icons.
Pidgin is a Gnome based, multi-protocol Instant Messaging client with support for IRC.. With Pidgin, you can manage your instant messaging accounts and IRC connections with one, graphical client.
CenterIM is a terminal based, multi-protocol Instant Messaging client with support for IRC. If you want the functionality of Pidgin in a text-based console, then CenterIM is for you.