A universal instant messaging (IM) program
See https://pidgin.im/about/ for more information. Pidgin is an instant messaging program which lets you log in to accounts on multiple chat networks simultaneously. It runs on Windows, Linux, and other UNIX operating systems. Pidgin is compatible with the following chat networks out of the box: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, MXit, Novell GroupWise Messenger, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. It is written in C and makes heavy use of GLib and GTK+. Finch is a command line instant messaging program. It also lets you log in to accounts on multiple chat network simultaneously, and it is compatible with the same chat networks as Pidgin. It is written in C and makes heavy use of GLib and ncurses.
Communication service (voip + chat)
climm is a portable, small, yet powerful console based, fully internationalized instant messaging client supporting the current v8 ICQ protocol and XMPP.
Gamra is a client of MRA chat for linux, by other words - Gamra is an IM client. MRA - Mail.RU Agent, the russian analog of ICQ. In the future, there will be an ICQ suppurt in GAMRA too.
Instant messenger on C for Linux platform