Z-Push (presumably Z is for Zarafa) is a FOSS implementation of the Microsoft Exchange ActiveSync protocol which is used to synchronize email, personal contacts and other items between a central server and a mobile device. Note the difference between this protocol, and an earlier (technologically unrelated) protocol named Microsoft ActiveSync.
Z-Push enables any PHP-based groupware package to become fully syncable with any ActiveSync-compliant device.
Currently, Z-Push includes four backends: the IMAP and the Maildir backend for e-mail synchronization, the vCard backend for contact synchronization and one for the Zarafa package which is sold by allowing full synchronization of E-mail, Calendar, Contacts and Tasks.
There is also a 3rd party project that implements a Zimbra Backend allowing Z-push to be used with a ZCS server (Including opensource edition)