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

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

Physics Central
A page on modern physics, such as quantum mechanics and black holes, and some mathematics.
http://www.physlib.com

Physics News Graphics
Diagrams (with captions) appearing in various AIP publications, sorted by category or by date.
http://aip.org/mgr/png

Physics Post
A tutorial and article driven community. Votes, science news and a discussion forum.
http://www.physicspost.com