- ChemSpider (Royal Society of Chemistry)
Open Access. Validated database of over 26 million substances from over 400 data sources. Links to primary research where available and verified.
- PubChem (National Library of Medicine)
Open Access. NLM database of compounds, substances, and bioassays, focusing on the biological activities of small molecules. PubChem provides substance information, compound structures, and bioactivity data for small molecules in three primary databases, PCsubstance, PCcompound, and PCbioassay.
- Common Chemistry (Chemical Abstracts Service)
Open Access. Contains CAS Registry file information for ~7900 general interest chemicals and 118 chemical elements. Searchable by chemical name, synonym, or CAS Registry Number. Provides basic data and Wikipedia link (if available).
- Kaye & Laby Tables of Physical & Chemical Constants (16th Ed.)
Open Access. The entire, unedited contents of the 16th edition (1995), containing tables of data, formulae, graphs and charts. Topics from fundamental constants to fibre optics, superconductivity to Raman spectroscopy, etc. Contents will be regularly reviewed and updated at the (UK) National Physical Laboratory.
- Spectral Database for Organic Compounds SDBS (AIST, Japan)
Compiled by the National Institute of Advanced Industrial Science and Technology in Japan. Contains mass spec, proton and C13 NMR, FT-IR, Raman, and ESR data for about 34,000 compounds combined. More information (including a disclaimer about accuracy) can be found here
(Mostly) Open Access. Data sheets for over 85,000 metals, plastics, ceramics, chemical elements, and composites. Search - Physical Properties (category + property ranges), Alloy Composition (category + property ranges) or Material Type, Manufacturer Name, Trade Name, Metal UNS Number
- International Critical Tables of Numerical Data, Physics, Chemistry and Technology (1st Electronic Edition)
Open Access. This classic and well-known reference was originally published from 1926 - 1930 for the National Research Council in 7 volumes. It contains an enormous amount of critical data on inorganic and organic compounds, and pure substances. Featuring physical, thermodynamic, mechanical, and other key properties, it is a major reference source used by those involved in chemistry, physics, and engineering.