Pidgin is a GTK2-based instant messenger application. It supports multiple protocols via modules, including AIM, ICQ, Yahoo!, MSN, Jabber, IRC, Napster, Gadu-Gadu and Zephyr. It has many common features found in other clients, as well as many unique features. Pidgin is not endorsed by or affiliated with AOL TimeWarner, Microsoft, or Yahoo.
Licq includes all the basic features of ICQ, like sending/receiving messages, chat, file transfer, contact list with pixmaps and user status, basic/extended user info, adding/editing users from within the GUI, user history, user groups, and new user registration. All commands and information are available through a simple and convenient tab dialog. Licq also has a completely configurable user interface with Skin and Icon pack support. It is written in C++ and comes with a GUI plugin using the Qt widget set. Other plugins are also available, like basic MSN support.
BitlBee allows users to talk to people on the major instant messaging and microblogging networks (including MSN, Google Talk, AIM, and Twitter) from within any IRC client by emulating an IRC server. Virtual channels are created with all of the user's contacts in them, who can be talked to in the channel or in a query. Also, one can participate in group chats and chat rooms like they were normal IRC channels.
climm (CLI-based Multi-Messenger) is a portable, small, yet powerful console-based ICQ and XMPP client. It supports password changing, auto-away, creation of new accounts, and other features that makes it a very complete yet simple client. It is able to send and receive messages from the ICQ network, list users that are offline and online (including their online status), search for users by email address, and look up basic (name, UIN, email) information. climm was formerly known as mICQ.
Gabber is a Jabber client for GNOME. Jabber is a flexible distributed Open Source instant messaging system. It also happens to allow communication with other instant messaging systems, such as ICQ, AIM, Yahoo!, MSN, and even IRC. Gabber aims to implement quite a bit of Jabber's functionality while remaining easy to use.
Kopete is a flexible and extendable multiple protocol instant messaging system designed as a plugin-based system. All protocols are plugins and allow modular installation, configuration, and usage without the main application knowing anything about the plugin being loaded. The goal of Kopete is to provide users with a standard and easy to use interface between all of their instant messaging systems, but at the same time also providing developers with the ease of writing plugins to support a new protocol. The core Kopete development team provides a handful of plugins that most users can use, in addition to templates for new developers to base a plugin on.
ickle is an ICQ2000 client using the GTK-- graphical toolkit. It aims to bring all the useful features of ICQ2000 to non-Windows users. It supports message/URL/SMS sending/receiving (including SMS delivery reports), status changes, reading away messages, and fetching advanced user info from a server. History browsing and lots of configurability in Settings are available.