Yamm module suite provides a client/server architecture for massive site to site data migration/update. This module suits for a lot of use case, site partial synchronization using another's site data, content copy and backup from one site to another site, incremental content copy from preproduction to production server, etc..
It only does Drupal to Drupal site synchronization. Now, it's able to synchronize any content type, node, taxonomy term and vocabularies and users.
This module is designed to be extendable, so any type of data could be synchronized using this module. Every piece of data is abstracted to what we call an Entity (as Drupal 7 does in core, both are not linked, but the funny thing is that both describe the same concept, and somehow are used in the same way).
This module relies heavily on OOP code (PHP 5.2 required).
It was inspired by the Deploy module.