Math Overflow and Mathematics StackExchange are wonderful websites, where one could interact with various people from various backgrounds, ask questions, reply to posts and so on. MO is for more advanced questions than MSE, and many professional Mathematicians spend a lot of time there.
Prof. Anthony Knapp's free to download Algebra textbooks. These include a textbook on elementary algebra, and another on more advanced Algebra topics.
Prof. Wolfgang Ziller's lecture notes on Lie groups, Representation Theory and Symmetric Spaces
Feynman's Lectures on Physics website. Prof. Feynman was a great physicist and a great teacher.
Prof. David Tong's various lecture notes on Physics. I really enjoyed reading many parts of his lecture notes.
Prof. Anthony Zee's homepage. I really enjoyed some of his lectures on Physics on youtube, and am considering purchasing his book on QFT at some point soon maybe.
Prof. Joe Polchinski's homepage. I highly recommend Prof. Polchinski's 2 volumes on string theory. Prof. Polchinski was kind enough to answer some of my (very basic) questions by email.