Is there a way to get a list of added packages so they can be re-installed after a clean installation (as opposed to upgrade)?

[Chris Green]

I'd really prefer to do a clean new installation occasionally rather
than doing a distribution upgrade.  On one system in particular I have
a number of issues which I think would probably be sorted out by doing
a clean installation.

It's easy enough to make a copy of (or use a different partition) for
/home so that that can be restored.

The difficult bit is to get back all the extra packages I have
installed in addition to the basic ones installed with Ubuntu/Xubuntu.  Is
there any reasonably clean and easy way to get a list of added
packages that one could feed into apt-get (or dpkg) to re-install all
the extras?

-- 
Chris Green