Wikipedia

The physics articles are mostly very good.

Hyper Physics

An amazing site, basically an encyclopedia for physics with javascript calculators and even a study forum.

Stack Exchange

It's not just for coding questions! Check out the

mathematics

and

physics

sections. Your questions have probably already been asked and answered here.

Wolfram Alpha

The free web-based version of their Mathematica software, with a natural language interpreter. Ask it to solve equations or evaluate integrals for you (it can solve both analytically and numerically), but there is a lot more it can do as well. It also can be used as an encyclopedia.

Paul's Online Notes

Particularly useful reference for solving differential equations.

Feynman Lectures on Physics

Caltech is hosting the 3 volume text online for free.

JPL's Basics of Space Flight

For those who think the topics in Chapter 13 are interesting. A very comprehensive introduction to all the things you need to consider when planning a space mission.

Frames of Reference

The 1960 film featuring Drs. Hume and Ivy.

Flanders & Swan, "First and Second Law"

A song about Thermodynamics. (Song itself starts at 2:20.) Great for tests, it gets stuck in your head right away. You have been warned.

Solar System Scope

A live view of the solar system, with info on each celestial body. Predict eclipses and conjunctions!

Stuff in Space

A realtime satellite tracker. Identify what's overhead. (Seems to work less well in Chrome...)

Powers of Ten

Structure at different scales.