How do I sync two folders in OS X Lion?

arRsync is a free OS X GUI to the rsync command line utility. It will get you started on the path to rsync enlightenment. You can save common sync operations to recall them at a later time for easy rsync. And it supports the common rsync use case scenarios: push A to B but leave stuff that's in B but not A in place, push A to B and delete stuff that's in B but not in A, make A and B look the same (with a conflict report).

If you are comfortable with the terminal, rsync is a great utility that will sync folders easily. Take a look at a tutorial here (the tutorial is on an "Everything Linux" website, but the rsync utility comes with both Linux and Mac OS X).

Example commands:

Copy/merge everything newer from SOURCE into DEST, do not delete anything:

rsync -va   /path/to/folder/SOURCE/  /path/to/folder/DEST/

Copy everything newer into DEST, and delete any items from DEST that don't exist in SOURCE:

rsync -va --delete   /path/to/folder/SOURCE/  /path/to/folder/DEST/

Be sure to include the trailing forward-slash / at the end of your folder paths.

A useful hint here is that you can drag folders into the Terminal window to have it insert the full path to that folder (or file), saving you lots of typing and typos.

Another donationware is SyncTwoFolders:

This program synchronizes two folders. The oldest files will be replaced by the most recent, and in certain cases, some files will be removed. Use the help tags to familiarize yourself with the options. Be sure of what you are doing when you start a synchronization. If in doubt, do a Simulation. This will show you what will happen if you choose to synchronize, but no files will be replaced or removed.

SyncTwoFolders is a free great app that's very useful and handy for Mac users. I use this to sync my GoogleDrive and Dropbox folders. Here is the link to setup SyncTwoFolders.

Actually you can schedule a sync. You have to create as setting of your own for ex. "mysync' by clicking the '+' sign next to 'Last Settings' drop down and saving it. This 'mysync' will appear in your window to the right when you click the '*' button above the 'Synchronize' button.

At the bottom of that sliding schedule window you can set the time interval at which it can sync.

You need to keep the application running in the background for it do scheduled syncing.