I am looking for information on software which backs up users' data
onto other computers belonging to friends and family etc, over the
internet.

I am interested in finding out whether this kind of software exists,
how (very briefly) it works and how much it costs.

I am aware of the existance of companies which provide this as a paid
service by allowing backing up onto a centralised server - this is
*not* what I am looking for. Also, I am not just talking about copying
and pasting files using eg a Windows Network - I am only interested in
software which does a little more than this.

I am interested in software for all machines/operating systems.

If no commercial products exist, I am also interested in any work
which is in progress by academic or corporate institutions.

Maybe this is something for you: http://backupforall.com/
With this program you can backup data to friends over internet; all
they have to do is setup an ftp server.

hi !

here is what you need..

ABC Backup 1.05 
Back up and archive your data to network disks and remote FTP servers.
OS: Windows (all) 
File Size: 802K 
License: Free 

http://www.download.com/ABC-Backup/3000-2242_4-10319037.html?tag=lst-0-4

- let's you backup your info over the Internet.
- is free.
- you can do more than a simple copy/paste.
- you can run this soft (as a client) on major windows based systems,
but you cannot run it from a Unix/Linux or Mac machine. as a server
you only need to download any ftp server for the platform you want to
and that's all!

hope I can solve your problem,
Emiliano.
=)