Centericq
- A console-based instant messaging system which supports the ICQ2000, Yahoo!, AIM, IRC and Jabber protocols.
Citadel
- An open source groupware platform which includes email, calendaring & scheduling, address books, bulletin boards, mailing list server, instant messaging, and multiple domain support.
Coccinella
- A cross-platform Jabber client. Key features include an integrated whiteboard, P2P file transfer, multi-lingual support, group chat, secured connections (TLS and SASL), and an open standard instant messaging protocol (can connect to any Jabber/XMPP instant messaging server). Site available in Dutch and English. Coccinella is the Latin name for ladybug.
Jabber Software Foundation
- Open XML protocol for an asynchronous, extensible instant messaging platform, and an IM network.
KXicq2, the ICQ client for KDE
- For KDE 2.0 and 3.0, but also works on a variety of Window Managers, including GNOME and Windowmaker.
Kopete
- A KDE instant messenger; a flexible and extensible multi-protocol system suitable for personal and enterprise use.
Licq Homepage
- An ICQ/AIM clone; also available in German. Latest stable release is 1.3.4, 2006-10-15.
Off-the-Record Messaging
- This instant messaging application allows private conversations by providing encryption, authentication, deniability, and perfect forward secrecy (if private keys are compromised, no previous conversation is compromised)
Pidgin- IM Client
- A cross-platform, multi-protocol instant messaging (IM) client for GNOME; multi-lingual support. Formerly called Gaim.
Psi Jabber Client
- A cross-platform, open source instant messaging software built on the Jabber protocol.
SIM
- An open source ICQ client; latest release is 9.4, 2006-06-30.