mIRC is a shareware IRC (Internet Relay Chat) client for Windows, developed by Khaled Mardam-bey. mIRC has many feature, such as multi-servers support, SSL connections, programmable function keys, custom CTCP replies, DCC and of course mIRC scripting.
More information about mIRC can be found at it's homepage, http://www.mirc.com/.
If you are interested in learning more about the IRC protocol and all that goes with it you could visit http://www.irchelp.org/.
|Link to download site and mirrors