These cool sites can help you learn anything from the philosophy of science to particle physics. Most are taken from Physics Today's WebWatch feature, and are catalogued below according to discipline. I know a great number of these fields overlap, so please be patient with my nomenclature!
Astrophysics & Cosmology
Astrophysics Data System - 2.8 million abstracts compiled by NASA
Big Bear Solar Observatory- An online compendium of solar images and information
From Stargazers to Starships - David Stern's educational site covering orbital and Newtonian mechanics, etc.
Hubble Space Telescope: New Views of the Universe
Radio Waves from Earth's magnetosphere and weather
Time Service Department of US Naval Observatory - Also info on global positioning system
White Dwarf Pulsations - from Travis Metcalfe at the University of Aarhus, Denmark
Biophysics
Animations of Processes within the Ear
Cure Cancer - Recruit your computer to help find candidate anti-cancer molecules!
Chemical Physics
IUPAC Compendium of Chemical Terminology
Physics and chemistry of ice cream
What is Mass Spectrometry? tutorial offered by the American Society of Mass Spectrometry
Chaos Theory
Complexity Digest at Penn State
Earth Sciences/Geophysics
Granular Volcano Group - Addresses the geophysics behind volcanoes
Interactive Experimentation and Physics by Computing
Analytical Electron Microscope real-time experiments
Visualizing Multidimensional Data by Computers
Lasers and Optics/Spectroscopy
X-ray spectra of elements on the periodic table - from Dr. Gregory Rech of Lawrence Berkeley Laboratory
Mechanics
From Stargazers to Starships - David Stern's educational website covering orbital and Newtonian mechanics, etc.
Nanomaterials and microengineering
Microelectromechanical systems (MEMS) clearinghouse
Nuclear Physics
World Nuclear Association and the Nuclear Energy Institute
Philosophy & History of Science
Advantages of Basic Research - Fascinating essays compiled in Beyond Discovery, published by the National Academy of Sciences
Bad Physics - A digest of oft-misunderstood physics principles, gleaned from the popular press.
Insultingly Stupid Movie Physics
Lars Onsager biography
Reading the History of Western Science - A list of 100 books you ought to read!
Physics quotes from Sci-Fi Novels!
The Joyce of Science: New Physics in Finnegans Wake
Safety & Health Information
Safety Manuals from Oklahoma State University - Includes what to do if a tornado approaches!
Solid State and Condensed Matter Physics
Crystal Structures from the Naval Research Laboratory
All You Wanted to Know About Silicon on One Page - by Dr. Kris Pister of UC-Berkeley