Irssi is a terminal based IRC client for UNIX-like systems. It's very popular amongst people who use IRC a lot. One of the reasons it's so popular is that you can run it on a remote server in a Screen, which allows you to always stay connected to your regular IRC channels. It's also a very customizable and extensible client with a lot of plugins and scripts available.
Howto/IrssiTips - Tips and tricks for the irssi IRC client
IRC/Gallery - Some screenshots of irssi and other IRC clients
weechat - Alternative IRC client