Soundation is a flash based multichannel sequencer, featuring a sampler, soft instruments and a built in loop library. If you want to lay out a simple house track it’s actually pretty capable – the default loops are high quality, and you can layer effects like phaser, distortion and delay on top of the track, all playing in real time.
Noteflight – Online Music Notation (free)
Flat.io – collaborate to compose music
NeoScores – Digital alternative to sheet music
MusicFirst – Online classroom for music education