Have been working on a web tool that I think is a bit different from the rest: It’s a music tracker that runs entirely in your browser!
It’s currently a beta-release, but with some patience and training you can get it to do wonderful things. So try it out now: http://sonantlive.bitsnbites.eu/
The most annoying limitation right now is the lack of a progress bar (you get kind of a progress indicator in Opera – but not in other browsers). Haven’t solved how to do the music generation in a web worker yet (for instance, I use a 2D canvas context in the synth, which I think isn’t possible in a Web Worker).