IMSLP: International Music Score Library Project from the Petrucci Music Library
Downloadable scores of almost any public domain piece (notably, every pre-modern piece from the Standard Repertoire page), as well as recordings!
Miscellaneous resource videos and infographics
Theory explanations, meme videos, and more to come!
Chrome Music Lab
This is a website with a variety of musical experiments, including a very easy-to-use music writing interface!
8-Bit Music Theory
This is a YouTube channel that does a fantastic job analyzing great music from video games. (Occasional slight language warning.)
Matthew Buckley
This is a YouTube channel that creates Line Rider tracks that synchronize with and depict classical pieces incredibly well!
Composition Vocabulary List
A list of words, concepts, and notation for understanding composition concepts, particularly those used by 8-Bit Music Theory
Downloadable assignments and explainers
Various worksheets and mini-articles about music theory concepts.