Well, in a 747 (and probably other widebodies as well) there is an unused space above the overhead bins of around 1 to 1,5 meters. This is where some airlines have their (8 person) crew rests. Besides this, the space from where the upper deck ends, till the tail is unused and you could fit at least 24 beds in there.

Only problem is that you are not allowed there. Airlines have not been able to certify the space for passengers, because evacuation of these spaces would take too long. Flight crew has had special training and is allowed. But I was quite surprised that these airplanes have so much unused space.