Jabber is an open XML protocol for the real-time exchange of messages and presence between any two points on the Internet. The first application of Jabber technology is an asynchronous, extensible instant messaging platform, and an IM network that offers functionality similar to legacy IM systems such as AIM, ICQ, MSN, and Yahoo.

I have successfully installed the Jabber server in a Linux environment and have had exposure to a number of Jabber clients, mostly on a Windows platform.