1. Eric Weisstein's Treasure Trove of Physics

Online encyclopedia of physics terms and formulas. Full searchable, and also browsable alphabetically and by topic. Part of Eric's Treasure Troves of Science.

http://scienceworld.wolfram.com/physics/

2. Physics and Astronomy Classification Scheme (PACS)

Hierarchical numbering scheme, developed by the American Institute of Physics, used to identify fields and sub-fields of physics.

http://publish.aps.org/PACS/

3. Physics Central

A page on modern physics, such as quantum mechanics and black holes, and some mathematics.

http://www.physlib.com/

4. Physics News Graphics

Diagrams (with captions) appearing in various AIP publications, sorted by category or by date.

http://aip.org/mgr/png/

5. Physics on MathPages

Articles, leaning towards a mathematical description, on a wide range of topics in physics.

http://www.mathpages.com/home/iphysics.htm

6. Physics Post

A tutorial and article driven community. Votes, science news and a discussion forum.

http://www.physicspost.com/

7. Physics Today

The flagship publication of the American Institute of Physics (AIP). Provides feature articles, news stories, analyses, book and product reviews, a searchable job database, and obituaries.

http://www.physicstoday.org/

8. Physics.org

Searchable database of physics resources from the Institute of Physics which matches a person's question, age and knowledge profile to handpicked sites. Also includes the Physics Life animation.

http://www.physics.org/

9. PhysLINK

Links to physics departments, physical societies, journals, job information, and other physics related information.

http://www.physlink.com/

10. Physnet: The Physics Departments and Documents Network

Serves information from physics departments and institutes worldwide: departmental information, locally stored documents, authoring tools, free access journals, jobs, conferences and education material.

http://physnet.physik.uni-oldenburg.de/PhysNet/

11. The Net Advance of Physics

MIT resource providing discussions at various levels of sophistication that cover all areas of physics.

http://web.mit.edu/redingtn/www/netadv/welcome.html

12. TheTangentBundle Physics Project

An experimental wiki-based collection of user-contributed articles on graduate level physics topics including subjects such as string theory, quantum field theory, mathematical physics and quantum mechanics.

http://www.physics.thetangentbundle.net/wiki/Main_Page

13. What You Want To Know About Physics

Summarizes in simple language, without advanced mathematics, the foundations of physics. The index gives direct access to selected topics.

http://kr.cs.ait.ac.th/~radok/physics/