Coccinella is a communication tool, something like a chat program combined with a drawing program, but with a lot of extra features.

Centericq is a text mode menu- and window-driven IM interface that supports the ICQ2000, Yahoo!, AIM, IRC, MSN, Gadu-Gadu and Jabber protocols.

Messenger:Mate is feature enhancing software that works in conjunction with AIM Classic.

Tkabber is a Free and Open Source client for the Jabber instant messaging system. It's writen in Tcl/Tk, and works on many platforms (tested on Linux, FreeBSD, NetBSD, Solaris, MacOS X and Windows).

Ayttm is an instant messaging (aka chat) client that provides all-in-one chat functionality for several major instant messaging services from one simple program.

BuddySpace is an instant messenger with four novel twists: (1) it allows optional maps for geographical & office-plan visualizations in addition to standard 'buddy lists'; (2) it is built on open source Jabber, which makes it interoperable with ICQ, MSN, Yahoo and others; (3) it is implemented in Java, so it is cross-platform; (4) it is built by a UK research lab, so it is 100% free with full sources readiily available.