KVIrc

KVIrc is a free portable IRC client based on the excellent Qt GUI toolkit. 

mIRC

The most popular shareware IRC chat client for Windows.

Irssi

Irssi is a terminal based IRC client for UNIX systems. It also supports SILC and ICB protocols via plugins.

xchat

XChat is an IRC chat program for both Linux and Windows. It allows you to join multiple IRC channels (chat rooms) at the same time, talk publicly, private one-on-one conversations etc. Even file transfers are possible. 

ChatZilla

ChatZilla is a clean, easy to use and highly extensible Internet Relay Chat (IRC) client, built on the Mozilla platform, which provides all the usual features: multiple servers, a built-in list of standard networks, easy searching and sorting of available channels, logging, and DCC chat and file transfers, plus easy customization with JavaScript plug-ins and CSS styling.