Chemical Information Sources - a thorough list of annotated links to major chemical info sources, originally created by Gary Wiggins at Indiana University
U.S. Chemical Safety and Hazard Investigation Board: A federal agency charged with investigating industrial chemical accidents. Website includes current and completed investigation reports, investigation digests, safety publications, recommendations, etc.
C2K - 2000 chemistry websites, compiled by Goodman Group, Cambridge University
ChemSpider (Royal Society of Chemistry) - a repository of searchable compound data, including structures and predicted properties. Includes nearly 25M compounds from almost 400 data sources. Chemists are invited to contribute their structures, spectra, and syntheses, and to participate in data curation.
TRAIL-Technical Report Archive and Image Library: a growing digital archive with a goal of providing persistent, unrestricted access to early U.S. federal technical reports (most prior to 1982). This is an academic collaboration that is not crosslinked to the federal government search engines. As of 2009, this held 1000+ reports in full text and is an excellent source for National Bureau of Standards Monographs.