Teoria.com is a comprehensive (and free!) music theory website I've just spent the last couple hours exploring. The ear training exercises are especially cool, with everything from recognizing basic intervals and triad inversions to 7 chords with all kinds of whacky extensions, melodic and rhythmic dictations, the whole nine yards. Really cool, and kinda addicting.
It's important realizing that music theory isn't a set of rules determining right or wrong, but rather a path towards unlocking our creative potential. Stevie Wonder needs advanced jazz progressions to communicate his art, whereas Bob Dylan, as he puts it, needs three chords and the truth. Adding sharp elevens to everything doesn't make you "better," and strumming G-C-D doesn't mean you're lacking - what the greats have in common's an authentic voice, and deep diving into our craft gets us closer to discovering our own.