AccessScience: McGraw-Hill's Encyclopedia of Science and Technology, online. Very useful for introductory information about topics in physics, and keyword searchable.
Net Advance of Physics: An online encyclopedia mainly of hot topics in physics. Articles are unsolicited but peer reviewed.
How Things Work: Physics in Everyday Life: A companion website to the textbook by Louis Bloomfield. "Call in radio show" website, is an archive of questions submitted by readers with answers by the professor. Answers are at a non-technical level.
ScienceWorld: Dictionary length definitions of scientific terms, created by Eric Weisstein--a professional encyclopediast at Wolfram (makers of Mathematica).