Info

jSyncer is a simple utility to synchronize the content of two file system directories.
Files found in both directories are copied overwriting the old copies, while files found in only one directory can be ignored, deleted or copied into the other one according to the user choice.

It's not "heavily" optimized therefore it can be quite slow when synchronizing directories with lots of files (and maybe it can suffer of memory usage problems).

I've never had problems using it but it's not guaranteed to work perfectly:
this program comes with ABSOLUTELY NO WARRANTY, USE AT YOUR OWN RISK!