Here is a few links that I really like, regarding physics:

Hyperphysics -
a very quick site to navigate, goes all-out with the hyperlink concept (covers a lot
of different tropics)

“A Radically
Modern Approach to Introductory Physics” – a new way to learn introductory phyics,
where the concepts is in there right context, regardless of the date of discovery
(Say; sir, do you want quantum mechanics with those waves?)

ScienceWorld – Wolfram (the guys, uhm,
the man, behind mathematica) is hosting a physics dictionary with some of the mathematical
concepts in physics (and since almost everything physics is mathematics, it tend to
be a good source)

MathWorld – And since you can’t say wolfram
without saying MathWorld, it just have to be mentioned, one of the best math sources
on the net.

HOW to BECOME a GOOD THEORETICAL
PHYSICIST – by  Gerard ‘t Hooft – A collection of good learing material,
by the nobelprize Laureate Gerard ‘t Hooft

Physicsforums – a generel forum for physics
and mathematics