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.
Psi IM — Qt-based XMPP client
Psi is cross-platform powerful XMPP client designed for experienced users. There are builds available for MS Windows, GNU/Linux and macOS.
A multi-protocol instant messaging app for Mac OS X
Adium is an Open Source, multi-protocol instant messaging application for Mac OS X. Adium supports AIM, ICQ, Jabber, MSN, Yahoo!, Google Talk, Bonjour, Gadu-Gadu, Novell Groupwise, SIP/SIMPLE (Text), and Lotus Sametime. Additionally, it supports all of these applications at once so, even if you have multiple IM accounts, you don't have to log into each account separately.
climm is a portable, small, yet powerful console based, fully internationalized instant messaging client supporting the current v8 ICQ protocol and XMPP.
PHP Jabber Bot open source. Minimal requirements: PHP > 5[CURL, ICONV, SOCKETS opened 5222 port], MySQL Included many plugins: google, horo, news, translate and more...
Java IM client with GUI based on JavaFX
Svembot is universal multiplatform Jabber Bot, which writed on python. Everybody can write his plugin and use svembot as "platform" for his bot. To write a plugin for svembot is easy, standart repository of svembot has many plugins.
This a new instant messenger for unix.
flManowar is simple jabber client with boards games support (battleship,checkers and reversi included)
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.
Communication service (voip + chat)
Instant messenger on C for Linux platform
nebohodimo IM is a Jabber client for Windows Mobile devices. It's written to be stable over unreliable mobile connection and easy to use.
Skysiss.com - IM bots and applications
Skysiss Skype, Jabber, Whatsapp IM bot assistants allow you to send messages to other IM. Replacement for your Weather application, Calculator and many others. Skysiss bots work directly from you favorite IM client.