Openfire is one of server application which implement XMPP (Jabber) protocol to serve chatting for client. This application is built in Java so you must install JRE on your server. Configuration of this server is so easy. You will have a web-based control panel for server configuration. This application is developed by Jive Software. It's support Windows, Linux, and Mac. You can download the installation file from HERE. Beside that there are many plugin such as for archiving or monitoring.
I want to show you how to install and make configuration of it on Linux.