Here's a selection of useful online and print resources for finding organic and inorganic chemical properties & reference data.
Remember also that the 'SciFinder,' 'ScienceDirect' & 'Web of Knowledge' databases are additional huge sources of chemical data:
A freely accessible collection of data relating to biologically-active small molecules, including compound structure, synonyms, descriptions, and bio-activity. Provided by the National Center for Biotechnology Information (NCBI).
Access note: Freely available to the public.
Database of substance properties, structures & toxicity from the National Library of Medicine.
Access note: Freely available to the public.
In addition to using the handbooks listed elsewhere on this page, here are some additional recommended sources of chemical and physical constants and laws.