ejabberd is a Free and Open Source distributed fault-tolerant Jabber server. It’s mostly written in Erlang, and works on many platforms (tested on Linux, FreeBSD, NetBSD, Solaris, Mac OS X and Windows NT/2000/XP).
I hadn’t ever heard of Erlang before. Some interesting things in there for building fast distributed fault-tolerent applications.
mmmm tasty jabber server