synchronising hard drives

hi,
A friend and i are looking for some synchronising software that can synchronise data on our hard drives. we live at different houses so it will have to work over the internet. basically we both have dedicated hard drives that we would like to synchronise and keep synchronised. so if he saves something to it then it will also appear on my hard drive too, and vice versa. we do not want to stream data from each other as this will be slow.
i was thinking that maybe every night, the harddrives resync.

i know this software exists as large companies back up all their data at night.

has anyone heard of such software and how to set it up etc, would i need a VPN? any advise and information appreciated

thanks