These sites are best for those who already have some familiarity with classical music.
András Schiff’s Beethoven lectures – A lecture series on Beethoven’s 32 piano sonatas. It’s interesting to see how a world-class pianist approaches a performance, making discoveries that would be lost on the casual listener.
Musopen – Download free mp3 files and sheet music. These folks have done a lot of legwork to make their content available without copyright restrictions.
Open Music Theory – An online “textbook” for those who want to dip their toes into music theory.
Petrucci Music Library – A huge collection of free downloadable sheet music. Here, you can find scores for the music you read about on Hello Classical.