I think that a lot of computer user are now used to the concept of copy on the “Cloud”, with tools like Dropbox or Ubuntu One, you can have the same file saved in multiple computers (also with a different operating system) and at the same time have a copy of it on the Cloud.
Usually you get some free space to start and if you need more you have to pay a monthly or yearly price, but these concepts where already there and available for Linux with Unison.
Unison is a file-synchronization tool for Unix and Windows. It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other.
The latest stable version, is 2.40.63. It is available as source code and precompiled binaries for Linux, Win32 and Mac OS X.