Knovel  Access to over 900 handbooks, in all fields of science and engineering.  Includes chemical data, encyclopedias, and handbooks, all word-searchable.  International Critical Tables and Handbooks of Physical Constants are found here.
College Physics  Good online basic textbook.  Excellent resource for books on all subjects.  Includes many of the Schaum's Outline series.
Handbook of Chemistry and Physics  Classic resource, now available online. 
E-print archive  Excellent free service, offering access to many important papers in all areas of physics and related fields.

Smithsonian Physical Tables 9th ed

UML Physics Reference Books

Some good reference books on physics include:

Besancon, Robert. Encyclopedia of Physics. 1985. REF QC 5 .E546 1985.

Dictionary of Physics. 4v. REF QC 5 .D53 2004.

Encyclopedia of Acoustics. REF QC 221.5 .E53 1997.

Encyclopedia of Atmospheric Sciences. 6v. REF QC 854 .E522 2003.

Encyclopedia of Astronomy and Astrophysics. 4v. REF QB 14 .E534 2001.

Encyclopedia of Nuclear Magnetic Resonance. 8v. REF QC 762 .E53 1996.

Encyclopedia of Physical Science and Technology. 18v. REF Q 123 .E4974 1992.

Encyclopedic Dictionary of Physics. 9v. & 5 supplements. REF QC 5 .E52.

Handbook of Physical Quantities. REF QC 61 .H36 1997.

Handbuch der Physik. 54v. REF QC 21 .H327 plus duplicates of most volumes in the Circulating collection.

Kaye, G.W.C. and Laby, T.H. Tables of Physical and Chemical Constants 1995. REF QC 61 .K3 1995.

McGraw-Hill Encyclopedia of Physics. REF QC 5 .M425 1993.

Thermophysical Properties of Matter. REF QC 171. P83 v. 1-8, 10,14[Master Index].