Common Chemistry is a gratis beta resource from the Chemical Abstracts Service which contains CAS Registry Numbers for approximately 7,800 chemicals of interest to the public.
… While not a comprehensive CAS Registry Number lookup service, Common Chemistry does provide a way to quickly and easily find names or CAS Registry Numbers for chemicals of general interest.
CAS has collaborated with Wikipedia in developing this resource and encourages you to use the Wikipedia link (when available) or other sources of general information on chemistry, to learn more about these chemicals.
See also comments by Antony Williams:
… We have already created the CommonChemistry.org Data Source on ChemSpider in case anyone wants to connect up records from ChemSpider with CommonChemistry as they are curating our dataset. …
It is unclear what licensing is on the data. I doubt it’s Open but that won’t matter to the majority of users…they are looking for a piece of information or to confirm something and are unlikely to be distracted by whether the data are Open or not…free access will suffice. …
See also our past posts on the Chemical Abstracts Service.