This page contains tables with material and equipment weights, etc. I'll add tables periodically as I create them.
The table below contains weights per square inch and density in pounds per cubic foot. It is worth noting not to focus just on density. For example, wall foam has a very low density, but is extremely soft and flexible. Balsa, on the other hand is quite stiff in the direction of the grain. What matters is the weight (grams) per square unit,in this case square inches (I need to put a metric column). The weight per square unit of a material just stiff enough for the purpose is what is important.