Technology, measurement, and standards information from the NIST, a non-regulatory agency within the Department of Commerce. Try their resources for Researchers: http://www.nist.gov/public_affairs/researchers.htm.
Three databases from the National Center for Biotechnology Information on chemical compounds, substances, and bioassays. The entries provide detailed information such as a general description, the molecular weight and formula, the structure, bioactivity results, chemical actions, detailed properties, and more.
Provides access to authoritative, selected science information from U.S. Government agencies, including research and development results. (Link goes to the Advanced Search. For the Basic search screen, click on their "Home" link in the menu.)
This easy to use site provides all kinds of information about the elements, including: background info, chemical and physical data, crystallographic, nuclear, electronic, biological, and geological information.
Wolfram Alpha describes themselves as a "Computational Knowledge Engine." Use their search mechanism to find comparisons of almost any quantifiable information. Follow their examples, or try it for yourself!