This is a neat site with density values for many common substances. You can search through solids, liquids, and metals. http://www.simetric.co.uk/si_materials.htm
An excellent site on chemistry and the periodic table. http://www.webelements.com
Another good periodic table site http://www.chemicalelements.com
and another http://www.chem4kids.com
and another http://www.chemcool.com
Finally, yet one more http://www.touchspin.com