Physics Links
http://www.livephysics.com/
A wealth of physics news, forums and teaching & learning tools.
http://phet.colorado.edu/
PhET: Free online science simulations.
http://www.quantum-physics.polytechnique.fr/
Quantum Physics Online.
http://www.compadre.org/
Resources for Physics and Astronomy Education.
Mathematica Links
http://www.wolfram.com/support/learn/
The Mathematica Learning Center.
http://www.wolframalpha.com/
Wolfram|Alpha: Computational Knowledge Engine.
Astronomy Links
http://hubblesite.org/
The official site for the Hubble Space Telescope.
http://www.zooniverse.org/home
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!
Miscelaneous
http://plato.stanford.edu/
The Stanford Encyclopedia of Philosophy. Contains entries spanning far
more that just philosophy.