When I'm working on a new project I often want to know the density of the materials I'm using, both for cutting speed reasons and weight considerations.  Here are the densities of some common materials you might choose to use.

• Acrylic, Polymethyl methacrylate (PMMA), "Plexiglas" - 73.4 lb/ft^3 : About 10 times stronger than glass, and crystal clear.
  Source: MakeItFrom.com - PMMA
• Balsa Wood - 7-9 lb/ft^3 : The density of balsa can vary greatly, because it is a naturally grown material.
  Source: Engineer Toolbox - Densities of Woods
• Extruded Polystyrene Blue/Pink Residential Insulation Foam - 1.30 lb/ft^3 : This is the blue or pink foam insulation you buy at the local lumber or home improvement store.  It typically costs about $30 for a 4'x8'x2" sheet.
  Source: Dow Chemical - What are the densities of STYROFOAM™ Brand Extruded Polystyrene Foam Insulation products?
  Source: ASTM C578 - 10a Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation
• High Density Polyethylene (HDPE) - 59.3 lb/ft^3 : HDPE is what kitchen cutting boards are commonly made of.  It comes in a variety of colors, and can be thermoformed.
  Source: MakeItFrom.com - High Density Polyethylene (HDPE)
• Medium Density Fiberboard (MDF) - 43.7-53.06 lb/ft^3 : MDF is dimensionally stable and easy to machine.  It is easily affected by moisture.
  Source: Wikipedia
• Oak - 37-56 lb/ft^3 : The density of oak can vary greatly, because it's a naturally grown material.
  Source: Engineer Toolbox - Densities of Woods
• Pine (white) - 22-31 lb/ft^3 : The density of balsa can vary greatly, because it is a naturally grown material.
  Source: Engineer Toolbox - Densities of Woods
• Polycarbonate, "Lexan" - 74.9 lb/ft^3 :  Up to 250 times stronger than glass, and crystal clear.
  Source: MakeItFrom.com - Polycarbonate (PC)
• Polystyrene - 65.5 lb/ft^3 :  Polystyrene is actually clear, though it can be bought in a rainbow of transparent and opaque colors.
  Source: MakeItFrom.com - General Purpose Polystyrene (GPPS or PS)
• Polyvinyl Chloride (PVC), "Vinyl" - 78.7 lb/ft^3 :
  Source: MakeItFrom.com - Polyvinyl Chloride (PVC or Vinyl)