Kahoot - create and play games and quizzes! Loads of music ones, or make your own. Students love it.
Quizizz - great classroom quizzes like Kahoot, but students can do as homework and you can get data!
BlendSpace - collate all of your digital content in one place - youtube, PDFs, dropbox, google drive, etc.
Vizia - pimp youtube videos by adding questions, polls and interaction.
Sibelius - the grandfather of notation software, and super intuitive for students to use.
MuseScore - it's free. Awkward to use at times but it's free!
NoteFlight - based online - no installation! Great collaboration, but limited notation functionality.
COMPOSITION AND TRANSCRIPTION
SoundTrap - write, record, master your music using browser based application.
HookPad - impressive songwriting tools for transcription, chord selection and melody writing.
Chordie - Play your midi controller, and see note names, notation, chord name and the guitar voicing!
iRehearse - great for learning music from a recording or transcribing music.
ORCHESTRAS AND INSTRUMENTATION
Philharmonia Orchestra | Instruments - detailed videos exploring all instruments of the orchestra.
New Zealand Symphony Orchestra classroom resources - great resources from New Zealand's own.
Music TALKS and DEMOS - interesting talks and demo videos.
Amazing ARRANGEMENTS - top arrangements and covers of songs.
Exploring INSTRUMENTS - amazing, interesting and weird instrument talks and demos.
Writing SONGS - videos and workshops on songwriting.
PERFORMANCES and PROJECTS - bizarre, interesting or incredible music performances and projects.