A gateway to scientific resources produced by the U.S. government. Click on the link for "Math, Physics and Chemistry" to see a listing of links to physics-related databases, government and government-funded research facilities and web sites.
A different type of "encyclopedia". Igor Ivanov has collected (mainly) scholarly links, preprint papers and bibliographies and arranged them in classified order by the appropriate subfield of physics. Offers many useful links. Not recently updated.