WeeChat is a fast, light and extensible chat client. It runs on many platforms, is modular, can support other protocols, is scriptable in 6 languages, fully-documented both in and
A fork to continue the development of the excellent XChat that is completely free on Windows and Unix systems.
Konversation is a fully-featured IRC client for Linux.
Pidgin is a cross platform instant messaging client that has multiple chat protocols including IRC. It has support for almost every other chat service available.
XChat is an advanced IRC client that is good for beginners as well as those users who like to get under the hood.
Polari is the GNOME IRC client.