Open Directory Project: Physics Links in categories, subcategories, and more; edited by subject specialists; organization is basis for directories used by Google and Yahoo.
The WWW Virtual Library: Physics "The WWW Virtual Library (VL) is the oldest catalogue of the Web, started by Tim Berners-Lee, the creator of HTML and of the Web itself, in 1991 at CERN in Geneva. Unlike commercial catalogues, it is run by a loose confederation of volunteers, who compile pages of key links for particular areas in which they are expert; even though it isn't the biggest index of the Web, the VL pages are widely recognised as being amongst the highest-quality guides to particular sections of the Web."
Scitopia Launched June 2007, federated search of the records of articles currently from 17 science and technology societies. Emphasis on physics, geology, and engineering. Basic and advanced search modes. Peer-reviewed journal articles and technical conference papers. Access to full-text depends on Brown University Library's subscriptions.
SPIRES-HEP Search "The SPIRES-HEP (High Energy Physics) database has been run by the Stanford Linear Accelerator Center(SLAC) since the late 1960's as a database of particle physics literature. SLAC also collaborates with other physics institutions around the world on SPIRES. In 1991 it became the first web-site in North America and now attracts around 50,000 searches per day from particle physicists." There are links to arXiv, HEPDATA, PDG (Particle Data Group), and SLAC books (SLAC Research Library catalog).