I used to use Imposition Publisher by Farrukh (http://www.farrukh.co.uk/) very heavily.
Sadly it doesn't seem to have been updated much. Their 'News' section has November 2008 as its most recent entry!
Shame really as their technical support was absolutely superb. Even managed to speak to a developer at midnight once! The joys of trying to impose Pagemaker 6 files