ChemSpider
ChemSpider is a free chemical structure database that provides fast access to over 100 million structures, properties, and related information, and is operated by the Royal Society of Chemistry.
By integrating and linking compounds from hundreds of high-quality data sources, ChemSpider makes it easy to find chemical data from diverse data sources that are freely available for online searching. Users can also add and manage data in a wikipedia-like fashion. Meanwhile, manual curation by the Royal Society of Chemistry continuously improves data quality.