ChemSpider
ChemSpider is a free collection of compound data from across the web, provided by the Royal Society of Chemistry (RSC). It aggregates chemical structures and their associated information into a single searchable repository. ChemSpider builds on the collected sources by adding additional properties, related information and links back to original data sources. ChemSpider SyntheticPages, CS|SP, extends this model to cover reactions, providing quick publication, peer review and semantic enhancement