MOOCs are online courses, many from top universities, some of them include a completion certificate or the equivalent. How to play the guitar is offered by UC Berkley. Other music classes are listed here, too.
The All-Star Orchestra, which is made up of "All-Stars" from various orchestras, produced content for Khan Academy. There is a wide-range of videos to watch from notes and rhythms, music history, conducting lessons, instrument demonstrations. This is a fantastic series with top musicians.