Atoms and Isotopes (see also Physical Constants, Units and Uncertainty)

• Atomic Weights and Compositions at NIST.
• Chart of the Nuclides (2004) at Japan Atomic Energy Research Institute
• Fundamental Physical Constants (speed of light, mass of elementary particles, etc., etc.).
• Ground levels and ionization energies for the neutral atoms.  Maintained by NIST.
• Table of Nuclides at the Korea Atomic Energy Research Institute
• WebElements^TM periodic table (Scholar edition)

General Physics

• Center for History of Physics maintained by the American Institute of Physics
• Java Applets on Physics by Walter Fendt (neat!)