Product review (3 stars): Lambro 4" Dryer Vent Seal
I think the Lambro Dry Vent Seal is a good solution, but there is a much less costly, more elegant solution available. It is the Jundas-Jafine wafer-type butterfly valve that is installed in any 4" diameter dryer vent. It is available from Lowes for $3.97 plus tax (November 2009). It probably creates less back pressure resistance to forward air flow than the Lambro does.