Compose Electronic Music, Right in Your Browser
audiotool.com
audiotool.com
No external controls, no external effects or synths (I know reason doesn't have them either, but everyone else does), no ability to sync to an external clock, etc.
While perfectly fine for putting together some beats, as anything other than a toy, it'd be hard to take it very seriously.
(PS. I wrote audio software back in the early 90's, including the first pattern based sequencer for Windows).
EDIT: I should add that I do think this is really cool and a nice app, BUT for what purpose? If musicians can't incorporate this into their workflow, then it's really just a toy from most musician/dj perspectives. And, they really haven't done anything that hasn't been done before, except for that it's targeted to the Flash runtime then where it really belongs: the desktop. I'm not certain an AIR version would even allow it integration via sync, rewire, etc.
The UI feels incredibly well thought-out, the virtual hardware artwork can only be described as a labor of love, and all the little UX things in a project that take way longer to get right than they seem, from the selection of device connections to way the cables lay around devices, feel like they've been iterated within a stone's throw of perfection.
If this were available on the desktop with MIDI support, it would be the first AIR app I'd actually pay money for.
It's too bad there's no way (that I know of, at least) for the Flash runtime to interface with MIDI drivers.
EDIT: Nevermind, I see you have to connect the outputs.