I have also compiled some cool sites from Physics Today's WebWatch feature.  Click here for a good sample from the last two years.  These are mostly research-oriented sites, but they can be used to further understanding in teaching advanced classes.

Professional Societies

Laboratories & Research

Education & Reference

American Association of Physicists in Medicine

American Association of Physics Teachers

American Institute of Physics

American Physical Society

American Society of Mass Spectrometry

Biophysical Society

Gordon Research Conferences online application

Institute of Physics (Britain)

National Science Teachers' Association (NSTA)

Optical Society of America

Resources of Scholarly Societies-Physics

Society of Physics Students (SPS) and its honorary, Sigma Pi Sigma

 

Argonne National Laboratory

Brookhaven National Laboratory

Brookhaven Protein Data Base - X-ray and NMR structures of biomacromolecules

European Organization for Nuclear Research (CERN)

European Space Agency 

Fermi National Accelerator Laboratory (Fermilab)

 Jet Propulsion Laboratory

Lawrence Berkeley National Laboratory

Lawrence Livermore National Laboratory

Los Alamos National Laboratory

NASA

Oak Ridge National Laboratory  and undergraduate research opportunities at ORNL

Sandia national Laboratories, California Site

Sandia National Laboratories, New Mexico Site

Stanford Linear Accelerator Center (SLAC)

BUBL Link - Physics Education.  An online database of Internet resources in physics education

Eric Mazur's website for ConCept Test materials.  Also see "Project Galileo" for downloadable resources

How Stuff Works - Well-written explanations of technology are useful for junior and senior high teachers.

LEGO physics projects! 

National Institute of Standards & Technology (NIST) - Get all kinds of data on constants, references, and units.  And if you're sick of your school's (and your students') clocks not ever being in sync, you set the standard for everyone with this cool connection to the NIST Cesium clock! 

Physical Science Resource Center - Devoted to teachers

Physics Applets - University of Oregon's suite of JAVA applets for use in physics, astronomy, and environmental science courses

Physics Central

Physics Today

Physics Departments Worldwide - A great database run by the University of Oldenburg's Department of Physics

Physics Links

Powers-of-Ten  Need a quick visual for a tutorial on the dimensions of the universe?  Here's one developed partly by Mike Davidson, photomicrograph whiz at FSU.  Also check out his incredible pics of the microscopic world!

Science Education Bookmarks

Optics for Kids