musictheory.net - "Introductory and intermediate music theory lessons, exercises, ear trainers, and calculators."
Open Music Theory - "a natively-online open...text and workbook for undergraduate music theory curricula...provides...a complete traditional core undergraduate music theory sequence (fundamentals, diatonic harmony, chromatic harmony, form, 20th-century techniques)...also several other units for instructors who have diversified their curriculum, such as jazz, popular music, rhythm, counterpoint, and orchestration. This version also introduces a complete workbook of assignments."
Sight Reading Factory - "...makes practicing the important skill of sight reading quick, easy, effective and fun! This cloud-based service allows you to customize and generate unlimited sight reading examples instantly, on-demand for students of all ages and abilities."