Physics Links

Preprints and Databases
arXiv.org — Formerly known as the xxx LANL archives, now hosted at Cornell university; the definitive source for preprints in physics and mathematics.
arXiv.org [hep-th/new] — The latest and greatest preprints in theoretical physics.
SPIRES HEP Literature Database
KEK Information Service System (KISS) for Preprints — Scanned preprints; might make your eyes hurt and but it saves a trip to the library.
Journals
Journal of Cosmology and Astroparticle Physics (JCAP)
Journal of High Energy Physics (JHEP)
Living Reviews in Relativity — An online journal offering reviews in all areas of gravitational physics.
Nuclear Physics Electronic
Online Journal Publishing Service (OJPS)
Public Library of Science
Organizations
American Institute of Physics (AIP)
American Mathematical Society (AMS)
American Physical Society (APS)
Institute of Physics (IoP)
Institutes and Research Centers
Aspen Center for Physics
Institute for Advanced Study (IAS) — Princeton, New Jersey.
Kavli Institute for Theoretical Physics (KITP) — At the University of California in Santa Barbara.
Los Alamos National Laboratory (LANL)
National Aeronautics and Space Administration (NASA)
SETI Institute Online — The Search for Extraterrestrial Intelligence. We've given up on Terrestrial Intelligence.
Reference
The NIST Reference on Constants, Units, and Uncertainty
The Particle Data Group
CelesTrak
Encyclopedias, Glossaries, and Biographies
Wikipedia — A open, free online encyclopedia.
ScienceWorld — Eric Weisstein's World of Science. Hosted by Wolfram Research.
PlanetMath
MathPages
Biographies
Prizes
The Nobel Prize Internet Archive
The Millennium Prize Problems — Need a quick million dollars? Solve one of these seven problems from the Clay Mathematics Institute in Cambridge, Mass.
The Ig Nobel Prizes — Recognizing outstanding achievements that cannot or should not be reproduced.
Popular Physics Sites
PhysicsWeb — Physics news, jobs, and resources. Published by the IoP.
Scientific American
Space Telescope Science Institute
The Official String Theory Web Site — I don't know how official it really is, but it has some good information (and some wrong information too).
The Particle Adventure — Award winning site on particle physics.
Pseudoscience
James Randi Educational Foundation (JREF) — One man's quest to cleanse the world of pseudoscience. It's a tough job, but somebody has got to do it.
The Talk.Origins Archive
The World of Richard Dawkins