IMIRC is an AOL Instant Messenger to IRC gateway. It connects to an IRC network as an IRC server, so it requires a local ircd to run. IMIRC creates nicknames for AIM users as they sign on, and converts HTML codes, which are used by AIM for formatting, to IRC's control code equivalents. Please note that you must either have your own working IRC server or you must find someone willing to let you link to theirs.
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.
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.
TiK is a pure TCL/Tk implementation of AOL Instant Messenger (AIM). It has most of the features the native Win32 and Mac clients have, plus some extras. It includes a graphical buddy list display, full font support, graphical emoticons, away messages, buddy pounce, buddy ticker, IM/Chat capturing, message broadcasting, a GUI preferences system, and much more. TiK's expandable nature allows the addition of new features through packages.
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.