Learning physics online?
Stanford has posted a bunch of theoretical physics courses by top physicist Leonard Susskind:
Classical Mechanics
Quantum Mechanics
Special Relativity
General Relativity
Cosmology
Statistical Mechanics
I'm a physicist but I still watched all these, just because Susskind is such a great teacher of physics. This is as good as it gets!
Since you say you're mostly interested in learning physics, not in a degree, there are lots of options. You say you have good knowledge of physics; if you want to learn particle physics, you need a good undergrad physics background plus some additional knowledge. I tend to think that reading textbooks is still the best way to learn the basics; if you want to learn particle physics, you might start with the book by Griffiths. If you find that you don't have enough background knowledge for it, then you would need to read other, more basic textbooks. Once you pass some threshold of background knowledge, though, you can learn a lot online. You could try searching arxiv.org for introductory lectures on various topics. In particle physics, for instance, you might search for the word "TASI", which is a summer school for graduate students to learn more about the field (the lectures are often written up and posted online). Most of this will assume you already know quantum field theory, though.
You can get a prepublication draft of Mark Srednicki's QFT textbook from his website (PDF file).
The Perimeter Institute has a one-year Master's degree program and videotapes all of the lectures. You can watch the videos from their archive.
List of recommended books and notes from the beginner level to advanced:
How to Become a Good Theoretical Physicist
Similar list on mathematics, parts of it may be useful for physics too:
How to Become a Pure Mathematician (or Statistician)
MIT OCW offers complete courses in Classical Mechanics, Electricity and Magnetism and others, designed for independent students. Tens of other physics courses are available, but may not be complete.