LarryMessenger is a simple to install and easy to use cross-platform instant messenger client. It has a nice graphical user interface, and allows people to communicate over a LAN with static IP addresses without the need for a central server. Furthermore, it provides functions to query the ICQ and AIM networks.
Gaim for Qtopia is an official port of the Gaim instant messenger. It is a multi-protocol instant messaging client for the Qtopia and OPIE embedded environments, compatible with the AIM (using the Oscar protocols), ICQ, MSN Messenger, Yahoo!, IRC, Jabber, Gadu-Gadu, Napster, and Trepia networks.
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.