CRC Handbook of Chemistry and Physics includes conversion factors, physical properties for organic and inorganic compounds, synonym indexes for organic and inorganic compounds, CAS numbers, Merck numbers, and Beilstein numbers for organic and inorganic compounds.
CHEMnetBASE includes information from the Combined Chemical Dictionary, Polymers: A Property Database, and Properties of Organic Compounds.
SciFinder Scholar searches properties data (physical, chemical, spectral) as well as links to relevant citations, as well as purchasing and regulatory information. You can search by keyword, topic, structure or other chemical identifier.
Reaxys includes the databases Beilstein (organic) and Gmelin (inorganic/organometallic), searchable by chemical structure, formula, physical property parameters, chemical reaction, as well as bibliographic data. You can also generate chemical structure from a name.
Knovel searches large collection of online handbooks (including tables of properties, some spectra)
These spectra from the U.S. National Institute of Standards and Technology can be searched by chemical name, molecular formula, CAS #, and selected physical property data.
Spectra: IR (16,000), MS (15,000), UV (1,600)
About two-thirds of the compounds in this database from the National Institute of Advanced Industrial Science and Technology (Japan) contain 6 - 16 carbon atoms. Lab measurement details are provided. The database can be searched by variety of parameters (compound name, molecular formula, molecular weight, CAS #, nmr peak positions, mass number, etc.
Spectra: ESR (2,000), 1H NMR (13,900), 13C NMR (12,000), IR (49,000), MS (21,800), Raman (3,500)
This covers spectra of 189 air pollutants mandated by Title III of the Clean Air Act. Spectra are grouped by chemical name, CAS # and molecular formula. Experimental measurement details such as cell path length, temperature, and concentration (ppm) are provided. Spectra are in spc format and are viewed using a free Windows-compatible viewer called ShowSPC.
Spectra: FT-IR (189 interferograms)