Fastest folder sync?

For web-hosted storage & sync, Dropbox is pretty fast.

New kid on the block is BitTorrent Sync (http://www.bittorrent.com/sync) which doesn't require uploading content to a server. It syncs folders between two computers directly. As such it's ideal for syncing folders quickly on the LAN.

 

You need to be more specific. Are you syncing from one Mac to another wirelessly? If so, the suggestions mentioned above are good ones. You could also look into Thunderbolt or USB 3.0 if you wish to do so through wires (always the preferred over wireless).

If you're asking about syncing one folder to another on the same Mac, you should be able to use simulinks or set up a folder sync method. I do not know too much about this but I use the Sync functionality built into Forklift by BinaryKnights i think.