OpenSRF uses Jabber (XMPP http://www.xmpp.org/) as it's backbone message passing protocol. XMPP is a standard, open protocol that uses a small set of XML tags to define the message structure.
There are numerous Jabber server implementations, many of which are open source and run on a variety of platforms. Initial development of OpenSRF relied solely on jabberd 2 (http://jabberd.jabberstudio.org/). Jabberd 2 has proven to be a fast and reliable server with which all OpenSRF configurations should run seemlessly. See jabberd for installation and configuration.
Recently we have added an in-house, streamlined Jabber implementation called Chop Chop, Jabber. Chop Chop is young, though it's proven fast and stable thus far. It currently only works, however, in single-domain OpenSRF configurations. Multi-domain support is planned for future versions of the server. see Chop Chop, Jabber for tips on building and running Chop Chop.