[nj5rq]

I currently use ERC, the GNU Emacs IRC client. Obviously, if you are not using Emacs already, you should not use it, so I recommend irssi. Pretty easy to configure, and with lots of options.

[coldpie]

Irssi was good, but Weechat has completely surpassed it.

[e12e]

And it has (had?) a decent slack plugin so you can avoid running slack...