My parents' ISP is blocking windows file sharing (port 139?).  But
they need to access files remotely.  I am looking for some free
software that will make "My documents" at home mirror "My documents"
at the office (which has its own IP).   I do not care if it gets files
real-time, or if it periodically synchronizes them.

This shouldn't be anything like an FTP program where they have to
manually connect -- I will set it up for them once, and from then on
it will just "work" automatically.  If they save a file at work, it
simply appears at home (and vice-versa).

I have seen services that advertise something like this, but they cost
money and/or have a limited storage space.