Can anyone recommend a GUI builder tool for creating DHTML web apps using AJAX to communicate with a web service backend? I'd like to avoid having to mess around with designing HTML, marshalling/unmarshalling data, checking for browser compatibility, etc. The tool should have a library of widgets that can be put into an application and hooked up to functionality, and be extensible enough to be able to define custom widgets. Of course free and/or open source is preferrable but I would consider proprietary tools too.
Also to what extent does the choice of GUI tool affect what platform or language I would use for the backend? If the GUI is just calling a web service then I should be able to use anything on the server-side to provide that interface but maybe some gui-side tools use a proprietary data exchange format that requires some specific server-side code?
Edit: I don't need a widget that can be dragged, I want an IDE that allows one to build a gui easily (i.e. a RAD tool).
On Dan's suggestion I took a look at GWT. I'd heard of it before but never looked at it that closely and it seems pretty interesting. Certainly it takes away the annoyance of pixel pushing and making things work in multiple browsers. I also tried searching for "gwt rad tools" and came up with a couple of results:
I'll take a look at these but if anyone's used something else let me know.