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.
Allows the modification of IRSSI's look and feel.
Pro Free and open-source
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.