Physical and Chemical Data

Data and Constants in Physicsweb
Databases list in ChemSoc
HEASARC (High Energy Astrophysics Science Archive Research Center)
NIST Scientific and Technical Databases
Physical Reference Data from the Physics Laboratory of NIST
CODATA (recomended values of the Fundamental Physical Constants)
SI Units (from the Physics Laboratory of NIST)
WebElements Periodic Table (author: Mark Winter, University of Sheffield)