Automatically sync select folders between two Macs over LAN
BitTorrent Sync does exactly what you want. It syncs over LAN by creating a "secret code" between the synced folders. You have the option to view your files on an iOS device through their free app. It's free and supports multiple platforms.

This is quoted from their website:


Sync and Share - Sync unlimited files between your own devices, or share a folder with friends and family to automatically sync anything.
Private and Secure - File transfers are encrypted. Your information is never stored on a server in the cloud and your data is protected by private keys.
Bigger is Better - BitTorrent Sync is specifically designed to handle large files, so feel free to sync original, high quality, uncompressed files.


I have been using it for couple months now. It's pretty much set it up and forget it.