HSDB - Hazardous Substances Data Bank  -  part of TOXNET - Toxicology Data Network

The periodic table of chemical elements at the International Union of Pure and Applied Chemistry (IUPAC)
IUPAC current (as of August 2010) website  -  IUPAC old website (discontinued May 2008)  -  Wikipedia lemma

The periodic table of chemical elements at www.ptable.com - highly interactive

The periodic table of chemical elements at Los Alamos National Laboratory

The periodic table of chemical elements at calculatoredge.com

The periodic table of chemical elements at webelements.com