[bemmu]

One way to make a site like this work would be if instead of hitting notes / recording in real time, you would interact with a tracker. Just like playing around with an 808, people would be able to click to set drum hits or notes. It would get played next time the track loops around (after a few seconds), by which time hopefully everyone would be synchronized.

[robhack]

Or the realtime could just be an illusion, like in plink http://labs.dinahmoe.com/plink/ (pretty fun little tool, even though you are quite limited because of the forced pentatonic scale and tempo)