I cannt find one online and I am suprised there isn't one... but is there an online Java GUI Layoutmanager at least?
WindowBuilder for Eclipse is phenomenal. One of the things that awesome about it is that you can hand modify its' generated code and it will pick that up without a problem.
What do you mean by an online GUI builder? If you want one that's a webapp, it's not likely you're going to find one.