XChat is a powerful IRC client with support for event-driven scripting in Perl, Python and Tcl, as well as C plugins. It's highly configurable and easy to use. You can change the keyboard shortcuts, create aliases, make scripts to ease automated tasks or useful bots. Although it hasn't been updated in over four years, it's still one of the best IRC clients for Linux and it comes included in every major distribution. To see a fork of XChat, have a look below at HexChat.

Kvirc is a beautiful, user-friendly, feature-rich client for KDE. It has its own scripting language and aliases manager, it supports add-ons (which can be downloaded and installed from within the application), and it has a very rich interface. It can fit both the advanced user due to the scripting possibilities and the beginner since it is easy to use and most functions can be accessed from the graphical menus or widgets. It also comes with smilies and support for themes, which are highly configurable. All in all, a very powerful and complex IRC client.

Quassel IRC is a client designed for the KDE desktop. The first time it starts it will display a wizard where you can choose your nickname, away message and network to connect. Quassel is pretty lightweight but still has many of the usual features found in IRC clients and rich configuration options.

HexChat is based on XChat, and takes development further. It comes with several improvements and new features, interface changes, new configuration options and bug fixes. As opposed to XChat, HexChat is in active development. It has support for scripting just like XChat, transparent background, background images, logging.

Konversation is a user-friendly client for KDE. It features smilies, a rich interface which should fit beginners to IRC. Konversation supports scripts (but not event-based ones), bookmarks, notifications, special characters selector, and it is highly configurable.

ChatZilla is probably the most popular IRC client for Firefox. Although it's a Firefox add-on, it's full-fledged and practically a program in its own right, and comes with features like networks list, DCC, essential IRC commands, aliases, auto-reconnecting, JavaScript scripts, appearance settings. ChatZilla has an abundant number of configuration settings which allow you to change its appearance and behavior. To start ChatZilla in the newer versions of Firefox, you can go to the Menu (top-right icon), click Customize and then drag the ChatZilla icon in your toolbar.