How can I find all the files which are not in packages in ubuntu?

I'm doing some backups, and want to only back up my data, not parts of the OS.  
I think I can just backup /home and /etc, but I want to make sure I didn't 
miss anything.  I'm also curious about the results.

I googled it, and found a solution for redhat/rpm:
   rpmfind --unknown

What would be the apt equivalent?  Pun intended.


Jeremy