Chemfinder 

Over 75,000 chemical compounds are indexed from over 350 web sites . 

CRC Periodic Table Online

Hazardous Chemicals Database

Comprehensive, peer-reviewed toxicology data for about 5,000 chemicals.

NIST Chemistry WebBook

IR, UV, mass, and vibrational spectra, as well as themochemical, thermophysical, and ion energetics data on nearly 36,000 species. Compiled by the U.S. National Institute of Standards and Technology.

Organic Syntheses

Full-text of all 70+ volumes of the Wiley publication Organic Syntheses. A compilation of selected syntheses of important organic compounds.

Periodic Table [ACS]

Periodic Table from the American Chemical Society.

SIRCh

Selected Internet Resources in Chemistry