musictheory.net does look good. neat design and somewhat interactive. Check out the "Analysis" lessons. Pretty good stuff there.
Couple of my faves:
http://www.tonalityguide.com/
http://www.schenkerguide.com/
Schenkerian analysis is pretty specialized and complex stuff - but quite powerful if you understand how & when to use it.
Originally Posted by: R. ShackleferdIn the beginning I absorbed some concepts of theory, but what I did understand was difficult to apply to actual spontaneous music or think about while actually playing, and have since pretty much ignored it to my own deficit. Of course now I realize the limits of "winging it" so to speak, and want to explore it further.
Just using this part as a springboard for some general thoughts.
There really is no way to "not use theory" when playing music. You can certainly not know the theory behind what you are doing. But the bottom line is that if you are playing music - notes, scales, chords, songs, etc. - you are in fact already using theory.
An analogy would be how as children we generally learn to speak before we know how to spell or be able to explain the rules of grammatical structure and the syntax of language. But ultimately if you want to improve at using language you must learn to understand the conceptual knowledge (literally, the "theory") that is the foundation for the application (literally, the "practice") of the words you are speaking.
An enormous amount of musical theory is contained in concepts such as "the major scale" or "a major chord". It is absolutely in any players interest to learn the theory behind the music they play. It will only improve your ability to think about and play music.