A MIDI enabled web chat made by Matt Helland offers a unique space to share live keys; via webcams and a whiteboard to teach instruments and theory, or collaborate. It uses WebMIDI and WebRTC to interact using a two-way interactive whiteboard for sheet music and other materials. It utilises his OpenMediaGallery (OMG) - a file-sharing social-network with an emphasis on composability. The project is open source with more details on the github page.
Try the app out here: https://bspoke.chat/apps/musicroom/piano.htm?t=demo&s=music