To my knowledge, Ejabberd (http://www.ejabberd.im/) is the parent, this is XMPP server which provide quite good features of open source, Whatsapp uses some modified version of this, facebook messaging also uses a modified version of this. Some more chat applications likes Samsung's ChatOn, Nimbuzz messenger all use ejabberd based ones and Erlang solutions also have modified version of this ejabberd which they claim to be highly scalable and well tested with more performance improvements and renamed as MongooseIM.
Ejabberd is the server which has most of the featured implemented when compared to other. Since it is build in Erlang it is highly scalable horizontally.
