Handbooks

Dictionaries/Encyclopedias
Physics Encyclopedia
The Laws List

Materials Safety Datasheets:
Cornell University
SIRI - Vermont
Interactive Learning Paradigms Incorporated

Constants and Formulae
NIST Reference on Constants, Units, and Uncertainty
Conversion of Units
Guide for the Use of the International System of Units (SI)
Integrator - Do integrals via the web.

Periodic Tables:
Center for Synchrotron Radiation Research and Instrumentation
Chemical Elements.com
Chemicool
WebElements

Physical Properties:
Acros Organics
ChemFinder.com
DIPPR
International Critical Tables
Knovel Critical Tables
NIST's Web Book
Spectrum's

Registry Numbers:
ChemFinder.com
ChemIndex Database

Spectra:
Acros Organics
LipidBank
NIST's Web Book
SDBS - Spectral Database for Organic Compounds
Sigma-Aldrich Chemical Catalog