Check out http://www.collabramusic.com http://www.collabramusic.com They are building a very cool product that allows you to jam with other musicians. Not quite

Check out http://www.collabramusic.com http://www.collabramusic.com They are building a very cool product that allows you to jam with other musicians. Not quite real time...but it is extremely close and allows you to create various "versions" of songs with different musicians/instruments recorded right into the browser.