The first periodic table on the WWW, created by Mark Winter of WebElements Ltd and University of Sheffield, England. Here you will find many pictures showing element structures and periodic properties and up-to-date information about new elements. (Note the cool image the author made using his name).
Created by the author of Webelements (see above) this site features a gallery of atomic orbitals and molecular orbitals, animated plots of wave functions and electron density, "Dot-density" plots of electron density, and plots of radial distribution functions.
C&EN Online is a weekly magazine published by the American Chemical Society (ACS) full of chemistry new from all over the world. It is available free to all ACS members. Access to some articles on this site requires a member number. To become a member go to https://portal.acs.org/portal/acs/corg/memberapp?_nfpb=true&_pageLabel=mapp_oma_application.
This site provides thermochemical, thermophysical, and ion energetics data compiled by the National Institute of Standards and Technology (NIST) under the Standard Reference Data Program.