An IRC client written by Timo Sirainen (aka cras). Home page at http://www.irssi.org/.
Irssi is a pretty nice IRC client for *NIXes and Windows (needs cygwin for Windows and appears to be slightly slowish compared to *NIX versions, though). Irssi supports stuff like plugins, scripting with language plugins (I think at least Perl and Python are supported, I have personally only used and seen Perl scripts), very nice configuration and config file format, theme support (though writing new themes is sort of crazy voodoo stuff), and nice management of IRC network and server lists (even better than what I hacked togheter for sirc back in the day =)... Also, I particularly like the automatic log file splitting (stuff from different channels go to separate files, arranged to directories by date as you see fit...). There are more features than anyone needs and it's all in a clean nice small package. As a whole, far less frightening beast than BitchX and far more interesting to script than ircII.
The name of the client seems to be a rather interesting play on words. The usual Finnish jargon for IRC is "irkki" (sometimes written "ircci", based on usual inflection of the word "IRC" - "ircissä", "irciin", the verb "ircata" and so on).
The letter "C" can be pronounced, depending on place, as either "k" or "s". (Some people seriously suggested, back ages ago, that Coca-Cola should be pronounced "sosa-sola"...) So, some people, jokingly or ignorantly, sometimes call IRC "irssi".