Unison works great for me. I use it to synchronize filesystems for two servers in remote locations over an OpenVPN connection. I also use it to sync my laptop up with my desktop (and then use rsync to backup the desktop copy to a remote server). It works well and can be automated although making the automation work securely can be a hassle.

For the two remote servers I have unison running every half hour via a cron script kicking off unison on server A to an instance of unison listening via sockets on server B. This works well until something kills the connection during a unison session and the unison listening on the socket dies and has to be restarted either manually or by a watchdog. For the laptop/desktop systems I kick off unison manually since I don't need to sync up often.

If you went the unison socket route on the server it would be fairly easy to set up unison on the laptop to run automatically via a script. The script could trigger when the laptop is able to ping the server do a test rsync for a specific file and if it passes follow up with unison).

The best part about unison is that in addition to doing two-way syncs is that it works with both Windows and Unix systems for those rare times that I need to work on a Windows box.