Irssi is a terminal based IRC client for UNIX systems. It also supports SILC and ICB protocols via plugins.
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Screen multiplexing and remote connection options with screen or tmux.
Pro Free and open-source
Allows the modification of IRSSI's look and feel.
Pro Perl scripting
Allows the user to create and use perl scripts to perform actions.
Pro Command line outputs can be sent to a channel or message
/exec -o figlet "Slant.co"
Pro Quick and easy setup
3 or 4 commands and you're ready to chat with all your old friends.
Pro Easy logging
Pro Persistent connection
Pro Has a plugin for encrypted communications
Con No GUI
Actually a plus for some, lower memory overhead.
Con Gives little direction upfront
Actually, Irssi.org has it's own GREAT documentation for new irssi users. Inline /help options are pretty well documented.