BBC BITESIZE - Gives a brief outline and very useful for revision.
Music Theory.net - This has lessons and tests to help you understand the basic principles of music theory. A very useful website. Music theory lessons and Tenuto are available as apps on Apple.
Last FM - A website to discover new music. Type in who you like and it will recommend musicians to listen to.
Get Revising - A fantastic website to help organising revision. Timetables, subject resources. General good advice.
Quizlet - For essential keywords and lists. Some fun games to help you remember the keywords.
Classic FM - Useful guides to explore on with Baroque, Classical, Romantic and Modern music.