A wealth of physics news, forums and teaching & learning tools.
PhET: Free online science simulations.
Quantum Physics Online.
Resources for Physics and Astronomy Education.
The Mathematica Learning Center.
Wolfram|Alpha: Computational Knowledge Engine.
The official site for the Hubble Space Telescope.
Zooniverse is a great site containing an incredibly huge amount of information on the universe. There are several "Citizen Scientist" projects that anyone can get involved in such as helping astronomers classify galaxies, measure features of the Milky Way, or search for extra-solar planets. A great site for kids too!
The Stanford Encyclopedia of Philosophy. Contains entries spanning far more that just philosophy.