Effusia Business Messenger
(commercial) Effusia Business Messenger is a cross-platform (Linux and Windows), centrally managed, internal instant messaging and presence communication tool for businesses of any size. The system comprises a client, a server, and an administration tool.
ionicChat is an Instant Messenger with a difference. It tries to connect users who are interested by a given topic to other users who are chatting on that given topic, dynamically and in real time.
JXTA Instant Messenger
JXTA Instant Messenger is an open-source IM application build on top of the latest P2P technologies. It is serverless & extesible and provides teams, projects & the internet with a tool to communicate in an innovative & secure way.
Kadu is a dynamically evolving instant messenger compatible with the Gadu-Gadu protocol. It can be run on all platforms supporting the Qt toolkit (except Windows).
Kde Instant Messenger is planned to support many different chatting protocols. Currently it's support is limited to TOC.
Kypeless is a fully decentralised chat program. It uses AES-256 encryption for data, and RSA public key cryptography for user identification. It relies on pre-shared public keys.
No-Name Instant Messenger Delta is a secure messaging client for Unix and Win32. It is currently in a pre-release state, but already offers secure peer-to-peer communication.
RedThat is a Reddit client for the Ubuntu desktop.
TorChat is a peer-to-peer instant messenger with a completely decentralized design, built on top of Tor's location hidden services, giving you extremely strong anonymity.
Tragic is a peer to peer IM and ICQ style software package.