Search by chemical name or CAS registry number.

By CAS, the American Chemical Society.

Sketch a chemical structure in the browser.
Also has built-in functions for producing Wikipedia ChemBox for the structure (cool!).

OPSIN is the (open source) standard for parsing IUPAC names.

OPSIN source code is on Github

This service works as a resolver for different chemical structure identifiers and allows one to convert a given structure identifier into another representation or structure identifier. It can help you identify and find the chemical structure if you have an identifier such as an InChIKey.

Does not seem to work right now, though.