The NCI CADD Group headed by Marc Nicklaus and colleagues has just launched the Chemical Structure Lookup Service (CSLS). This web-based system allows one you to locate chemical structures in over 70 different public and commercial data sources. Stored within the system is information on over 30 million chemical structures and it provides a simple search interface for looking up chemicals by specific structure as well as by parent structure, and by various identifiers.
There are two mirror sites: http://cactus.nci.nih.gov/lookup/ and http://cholla.chemnavigator.com/lookup/