Theory
Tom Pankhurst's Guide to Schenkerian Analysis - Home Page.
SchenkerGUIDE.com aims to explain both the practical basics and the rationale behind Schenkerian analysis and serve as an introduction to Schenker's influential and controversial theory of tonal music.
http://www.schenkerguide.com/
Our Purpose
Arts, Music, Theory - The Society for Music Theory. Includes a database of journal article from the SMT Journal.
https://societymusictheory.org/
Good Ear - Online Ear Training Site
Arts, Music, Theory - Good Ear. Online ear training site.
https://www.good-ear.com/
Ear Training Exercises - Ear Training Pro
Learn to identify the intervals between notes by ear, starting from beginner level exercises, though to advanced exercises identifying the interval between notes played at the same time.
http://www.eartrainingpro.com/