MattisManzel:
A must is moon-edit. http://me.sphere.pl/indexen.htm. Not directly musicially, but increddible collaborative - kinda text-jam-session - and even has a rudimentary sequencer integrated. Check sphere.pl (the most active of few server up to now) and look for a document you are not alone on, a bit patience. This is supernewsuperhot. cu on moonedit.