Irssi is a highly customizable text based IRC client. It supports coloured text highlighting, windows, multiple servers and is highly extensible via Perl scripting.
It is not very user-friendly and well documented if you don't know how it works, but once you get used to it, it is really great to use. In that way, it's in the same league as vim and mutt: power-user tools. If you don't want to spend the time on learning irssi, you're better off with xchat or smuxi and the like. On the other hand, irssi can be used from GNU Screen, and you can connect to it from any other PC.