The arXiv and other sites

* arXiv.org and its many mirrors.
* Arxiv Structure site at xstructure.inr.ac.ru.
* PhilSci Archive at the University of Pittsburgh.

Good sites on many of these topics

* The Scholarpedia Encyclopedia of Physics.
* David Brown's GRwiki at North Carolina State University.
* David Darling's Encyclopedia of Science.
* Einstein Online at the Max Planck Institute for Gravitational Physics.
* Rod Nave's Hyperphysics.
* The nLab, a "wiki-lab" on Mathematics, Physics and Philosophy, especially from the n-point of view.

Some other search sites that provide information on many of the topics in this site (in alphabetical order)

* Answers.com.
* Factbites.com.
* Google Scholar.
* Wikipedia.org.

Some blogs on related subjects

* Steve Sekula's Adventures of My Pet Hamster [@ pw(08)nov ... Oct 2015 ...].
* Clifford Johnson's Asymptotia [@ pw(07)jul ... Oct 2015 ...].
* Robert Helling's atdotde [2004–2014].
* Sabine Hossenfelder Backreaction [@ pw(07)jun ... Oct 2015 ...].
* Michael Nielsen's blog [@ pw(08)aug ... May 2015 ...].
* Steinn Sigurdsson's Cat Dynamics [@ pw(08)feb ... Oct 2015 ...].
* Jennifer Ouellette's Cocktail Party Physics [@ pw(07)aug ... Oct 2015 ...].
* IYA2009's Cosmic Diary [@ pw(09)mar ... Oct 2015 ...].
* Peter Steinberg's Entropy Bound [@ pw(08)jun ... Oct 2015 ...].
* Andrew Jaffe's Leaves on the Line [@ pw(08)sep ... Mar 2015 ...].
* Gordon Watts' Life as a Physicist [@ pw(08)apr ... Jun 2015 ...].
* Matthew Nobes' and Georg von Hippel's Life on the Lattice ... Sep 2015 ...].
* Doug Natelson's Nanoscale Views [@ pw(08)feb ... Oct 2015 ...].
* Peter Woit's Not Even Wrong [@ pw(07)may ... Oct 2015 ...].
* Sean Carroll's Preposterus Universe [... Oct 2015 ...].
* David Horgan's quantumtetrahedron [... Oct 2015 ...].
* Physics section of Scientific Blogging [... Oct 2015 ...].
* Richard Jones' Soft Machines [@ pw(07)nov ... Oct 2015 ...].
* Pamela Gay's Star Stryder [@ pw(08)may ... Oct 2015 ...].
* Dave Bacon's The Quantum Pontiff [@ pw(08)jan ... Mar 2015 ...].
* Luboš Motl's The Reference Frame [@ pw(07)oct].

For people interested in other points of view on some of the topics covered here

* Dimi Chakalov's God does not play dice.
* The alternative e-print archive viXra.org.
* The General Science journal site gsjournal.net.