Multiplayer Piano is an online, full 88-key piano you can play alone or with others in real-time. Plug up your MIDI keyboard, MIDI in and out are supported. You should be able to hear some seriously talented piano players performing here! Join in or just chat and listen.
For good performance, Chrome is highly recommended. Firefox also supports the requisite Web Audio API, but performance may not be as good. Chrome has Web MIDI.
Of course, first you need to Enable Javascript or it won't do anything...!