Material safety data sheets (MSDS) is a perennial search favourite on the Chemspy site. The site has its own MSDS section and a javascript widget you can even add to your own site to allow your visitors to search for chemical MSDS sheets (yes, I know it’s a tautology on two counts, chemical material safety data sheets sheets, but like PIN number and “ATM machine”, the phrase is in common parlance.
Anyway, follow the link to search for MSDS on ChemSpy and here if you’d like to add the MSDS search toolbox (feel free to edit the script once you’ve saved it to your server to fit your site’s requirements).
Meanwhile, an offsite resource that looks promising for Chemical MSDS sheets is the obviously named MSDS Search, which duplicates some of the ChemSpy resources but adds a few new ones too.