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 (http://www.webelements.com/)
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.
ChemSpider.com
(http://www.chemspider.com/)
CRC Handbook of Chemistry and Physics
(SEL Desk QD 65 .H3)
Merck Index
(SEL Desk RS 356 .M4)
Aldrich Catalog
(SEL Desk 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.
General
Kirk-Othmer Encyclopedia of Chemical Technology
(Ref TP 9 .E658)
CHEMnetBASE
http://www.chemnetbase.com/
Knovel.com
(http://www.knovel.com/)
Organic
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)
Inorganic
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)
ATSDR's Toxicological Profiles
(http://toxprof.crcpress.com/)
Integrated Spectral Data Base System (SDBS) http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng
This
site, from the National Institute of Materials and Chemical Research in
Japan, contains full spectra and, in many cases, peak assignments for
33,000 compounds, including about 24,000 mass spectra, 13,000 13C
NMR, 14,700 proton NMR, 51,100 IR, 3,500 Raman and 2,500 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.
Sigma-Aldrich Online Catalog
Contains
thousands of FT IR and FT NMR spectra for compounds in the catalog.
Searchable by chemical name, formula, CAS Registry Number.
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.
(Ref QD 95 various)
Spectroscopic Tools (http://www.chem.uni-potsdam.de/tools/index.html)
This
site, based at the University of Potsdam, 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.