1. Openfire

... instant messaging, XMPP (also called Jabber). Openfire is incredibly easy to setup and administer, but offers rock-solid security and performance. Visit Openfire! ...

2. Pidgin

Pidgin is an instant messaging program for Windows, Linux, BSD, and other Unixes. You can talk to your friends using AIM, ICQ, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise ...

3. Spark

Spark is an Open Source, cross-platform IM client optimized for businesses and organizations. It features built-in support for group chat, telephony integration, and strong security. ...