Besides the CRC Handbook of Chemistry and Physics (see below), there are a number of sources which specialize in units of measure and other basic information for scientific calculation. Examples include:
For Good Measure: A Complete Compendium of Intenational Weights and Measures (Ref QC 88 .J6)
Provides extensive conversion factors for metric to and from other units, and includes an entertaining assortment of obscure and obsolete units.
Conversion Tables of Units for Science & Engineering (Ref QC 94 .H67 1986)
Conversion factors for a wide range of useful units, arranged in tabular form.
Handbook of Physical Calculations, 2nd ed. (Ref QC 61 .T85 1983)
Mainly oriented toward physical and engineering calculations, this work also has extensive tables of units and physical constants.
NIST Reference on Constants, Units and Uncertainty (http://physics.nist.gov/cuu/index.html)
This National Institute of Standards and Technology has a searchable and browsable list of the important physical constants with bibliography, as well as a thorough description of the SI system of units and a description of methods of expressing degree of uncertainty in measurements. An excellent reference site.
The Laws List (http://www.alcyone.com/max/physics/laws/index.html)
This collection of laws, rules constants and definitions of concept in physics, compiled by Erik Max Francis, includes a number of interest in physical chemistry, e.g. the gas laws, Rydberg formula, etc.
WebElements is a hypertext-linked collection of property data on the first 112 elements (and some beyond) including (where available): general, chemical, physical, nuclear, electronic, biological, geological, crystallographic, reduction potential, isotopic abundances, electronic configurations, ionization enthalpy data and additional textual information, especially on the history of the elements.
CRC Handbook of Chemistry and Physics
(SEL Desk QD 65 .H3)
(Ref Desk RS 356 .M4)
(TP 202 .A48, also uncataloged copies)
Wikipedia Chemistry Portal (http://en.wikipedia.org/wiki/Chemistry_portal)
Wikipedia has a project dedicated to brining quality chemical information to the online collaborative encyclopedia. You can follow links from the portal, or search by chemical name in the regular Wikipedia search box. For common chemicals (some 1000 substances), you can also search by name, synonym or CAS Registry number at Common Chemistry (http://www.commonchemistry.org/), a cooperative project of Wikipedia and Chemical Abstracts Service. Wikipedia articles on chemical substances typically have basic physical property data, but availability varies by substance.
Kirk-Othmer Encyclopedia of Chemical Technology
(Ref TP 9 .E658)
CRC Handbook of Data on Organic Compounds (HODOC)
(Ref QD 257.7 .H36)
Handbook of Physical Properties of Organic Compounds
(Ref QD 257.7 .H374 1997)
Dictionary of Organic Compounds, 6th ed. (Ref QD 251 .D5 1996)
Dictionary of Inorganic Compounds
(Ref QD 148 .D53 1992)
Encyclopedia of Inorganic Chemistry
(Ref QD 148 .E53 1994)
Sax's Dangerous Properties of Industrial Materials
(SEL Ref T 55.3 .H3 L494 2000)
Hazardous Substances Resource Guide
(SEL Ref Desk T 55.3 .H3 H344 1993)
Spectral Database for Organic Compounds (SDBS) http://sdbs.db.aist.go.jp
This site, from the National Institute of Materials and Chemical Research in Japan, contains full spectra and, in many cases, peak assignments for 34,600 compounds, including about 25,000 mass spectra, 14,200 13C NMR, 15,900 proton NMR, 54,100 IR, 3,500 Raman and 2,000 ESR spectra. The database is searchable by compound name, CAS Registry Number, molecular formula and NMR, IR or MS peaks. The database is free to the public, but users are asked to download no more than 50 spectra per day without specific permission of the site owners.
SpectraBase is a freely available collection of spectra, covering hundreds of thousands of organic, organometallic and inorganic compounds. Spectra include proton NMR, heteronuclear NMR, FTIR, transmission IR, Raman, UV-VIS and mass spectra, plus basic property data, though not all spectra types are available for all compounds. Peak assignments are available for NMR spectra. The collection may be searched by chemical name, CAS Registry Number or InChi key. Only one spectrum may be displayed at a time. Note that (free( registration is required, and there are frequent advertisements displayed for the database producer, Bio-Rad.
Aldrich Library of Infrared Spectra, 2nd ed. (Ref QD 96 .I5 P67 1981)
Sadtler Handbook of Infrared Spectra (Ref QC 453 .S73 1978)
NIST Chemistry Webbook http://webbook.nist.gov/
Among other data, NIST Chemistry Webbook has IR spectra for over 16,000 compounds, which may be searched in a variety of ways, displayed and printed.
Sadtler Handbook of Proton NMR Spectra (Ref QC 490 .S23)
Sadtler Guide to Carbon 13 NMR Spectra (Ref QC 762 .S28 1983)
Aldrich Library of NMR Spectra (3 volumes) (Ref QC 762 .P69)
Aldrich Library of 13C and 1H FT NMR Spectra (3 volumes) (Ref QD 96 .F68 P67 1993)
Handbook of Proton-NMR Spectra and Data (10 volumes + index) (Ref QC 762 .H33 1985)
Eight Peak Index of Mass Spectra, 3rd ed. (QC 454 .M3 M41 1983)
This seven part set gives the eight most abundant ions in 66,720 mass spectra, indexed by molecular weight, elemental composition and most abundant ions.
NIST Chemistry Webbook http://webbook.nist.gov/
Among other data, NIST Chemistry Webbook has mass spectra for over 15,000 compounds, which may be searched in a variety of ways, displayed and printed.
Spectroscopic Identification Tools
Spectroscopic Tools (http://www.science-and-fun.de/tools/l)
This site allows the user to plug in IR peaks in wavenumbers, proton NMR peaks or mass spectral peaks and retrieve a list of the functional groups which would generate those peaks. It also has a 13C NMR database, searchable by peak or chemical name fragment, which can retrieve structures, spectra, peaks and peak assignments. Some features require the Chime browser plugin.