American Chemical Society
http://portal.acs.org/portal/acs/corg/content
Chemical Heritage Foundation
http://www.chemheritage.org/explore/explore.html
ChemWeb.com
http://www.chemweb.com/
General Chemistry Case Studies
http://www.chemcases.com/
NIST Chemistry WebBook
http://webbook.nist.gov/chemistry/
Periodic Table of Videos
http://periodicvideos.com/index.htm#
PubChem
http://pubchem.ncbi.nlm.nih.gov/
Reactive Reports
http://www.reactivereports.com/
Science.gov
http://www.science.gov/
United States Patent and Trademark Office
http://www.uspto.gov/
Visual Elements Periodic Table
http://www.rsc.org/chemsoc/visualelements/pages/pertable_fla.htm
WebElements Periodic Table of the Elements
http://www.webelements.com/index.html
What's that Stuff? from C&EN
http://pubs.acs.org/cen/whatstuff/stuff.html