KVIrc is an open code IRC client, totally free, portable and very complete, that was initially conceived to be used with Linux, and that now can be used with Windows too.
There are numerous functions and characteristics. It supports themes, transparencies, unread text marks, simultaneous connections to various servers, notification lists, ignore lists, lag and delay measurer, complete system of logs and reports, support for Unicode, SSL connections, UTF-8 codification, SDCC protocol, etc.
Another interesting KVIrc quality is that you can code conversations and file transfers, to assure you that unwanted attention will not gain access without permission. Also, you can widen its functions via modules if you want to.