Updated: May 24, 2008
Made in GTK, XChat is a complete IRC client featuring a powerful scripting interface through Perl, Python or TCL, plugins interface in C, a logging system which uses time format specifiers and many more. It is probably the best GUI client for Linux. On the official website are available many scripts for making XChat behave the way you like it, including now playing scripts or trivia bots. Online documentation is also abundant, the official documentation site offering help for all the scripting languages it supports. Can be embedded in the system tray and includes customizable notifications for events.
Konversation is a user-friendly IRC client using KDE, highly configurable, but with no support for events scripts, at least not in the current version (1.0.1). For example, you can't make a script to auto-greet somebody when he joins a channel. Konversation is perfect for someone who just needs a friendly IRC client and doesn't want to be bothered with scripting it. Can be embedded in the system tray with the possibility to flash the icon (or the taskbar) when you receive messages.