A comprehensive site for beginner learners, offering interactive music theory lessons for free, as well as paid-for apps for iOS (Apple).
An extensive resource of online theory instruction, established in 1997 by José Rodríguez Alvira from the Conservatory of Music of Puerto Rico. Includes interactive tutorials, exercises, articles and other reference materials.
(which offers an online ear training software) offers a free introductory music theory course which starts with the very basics..
Music Theory Videos
Stephen Wiles, a London-based music teacher, has developed a series of music theory videos loosely based on the first five grades of the Associated Board (ABRSM) syllabus.