The granddaddy of all free chemistry databases. Search over 8 million compounds by a variety of criteria. Although some PubChem records are linked into the primary literature through MeSH, most are not. But this doesn't seem to be PubChem's true calling. Instead, PubChem may well evolve into the world's largest online collection of molecular data sheets. Increasingly, the other databases in this list are cross-referencing their entries into PubChem. PubChem's entire database can be downloaded by FTP. CAS Registry are correct to see PubChem as the first real competition they've had in decades.