Irssi is a modular IRC client that currently has only text mode user
interface, but 80-90% of the code isn't text mode specific, so other UIs could
be created pretty easily. Also, Irssi isn't really even IRC specific anymore,
there's already a working SILC module available. Support for other protocols
like ICQ could be created some day too.

WWW: http://www.irssi.org/