Thanks to a 'feature' of LEGO's new replacement parts portal, discovered by the team over at The Daily Brick
, Brickset's set inventories now include quantities
This makes the likes of http://www.brickset.com/detail/?Set=3920-1#ui-tabs-4
much more useful, and of course opens up all sorts of other possibilities I've only just stared to explore on pages such as this http://www.brickset.com/detail/inventory/?Set=3920-1
Inventories for about 4600 sets are available, although some are still not complete. A future development will probably be to enable people to verify and complete them, armed with the inventory sheet at the back of the instructions.
I hope you find it useful...