InstantMessaging - Debian Wiki

Instant messaging (IM) technology is a type of online chat that offers real-time text transmission over the Internet.
IrcClients - clients for the IRC protocol
JabberClients - clients for the Jabber/XMPP protocol

There are many other alternatives:

Graphical:

https://screenshots.debian.net/package/Pidgin

pidgin : multi-protocol client (plugins)

kopete : multi-protocol client

kadu : Gadu-Gadu client

psi : Jabber client

tkabber : Tcl/Tk Jabber client

thunderbird : An email client with Instant Messaging features

Text/console interface:

https://screenshots.debian.net/package/finch

finch : multi-protocol client

ToDo: include video/audio-conferencing software/servers