Openfire is an open-source Jabber (XMPP) server that provides an easy way to offer and maintain internal instant messaging (including voice and video chat) for your company or organization. There are server plugins available which can add archiving and statistics capabilities as well as message of the day and multi-user chat (MUC) bookmarks.

Openfire conveniently supports LDAP and Active Directory authentication as well as SSL for encrypting communications. If you require high availability, you can enable clustering to eliminate a single point of failure. Common databases like MySQL, Microsoft SQL, and PostgreSQL are supported, or you can use Openfire's embedded database.

While the developers offer a web-based (SparkWeb) client and install-based (Spark) client, it's possible to use any IM client that supports Jabber (Adium, Pidgin, iChat, etc.). Openfire runs on Linux, Mac OS X, and Windows and since it's Java based, it will require that you have a JRE installed.