Ejabberd

ejabberd is a free and open source instant messaging server written in Erlang.

ejabberd is cross-platform, distributed, fault-tolerant, and based on open standards to achieve real-time communication.

ejabberd is designed to be a rock-solid and feature rich XMPP server.

ejabberd is suitable for small deployments, whether they need to be scalable or not, as well as extremely big deployments.

http://www.process-one.net/en/ejabberd/