Multiplayer Piano is an online, full 88-key piano you can play alone or with others in real-time. MIDI input is supported via midibridge-js (java) so you can really play on this thing. 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. Other browsers do not provide the requisite Web Audio API, with the exception of a very recent Nightly Firefox build which is still in beta, and you will get poor performance using a Flash fallback.