Memo.ai - Simple and elegant notes app on your Mac
AMPLI.FI - Live online music community. Twitch for tunes.
Music Memos by Apple - An app to capture your song ideas
Bandlab - Capture, nurture, and collaborate on song ideas in the cloud
Better Notes - Simple notes app that ties notes together with #hashtags
Twitch Creative - Watch artists create. Twitch expands into the arts.