Can you recommend an open source AJAX application-building development tool that will help me build browser applications with a drag-and-drop interface?

Tibco's General Interface Builder is open source and available under a BSD license. You can download the development environment. The software comes in a zip file that you simply unpack onto your Windows system and then run the GI_Builder.hta file in the installation directory you chose to launch the application.

General Interface Builder is a browser application that provides a reasonably friendly drag-and-drop-style interface for laying out your own browser application using a library of provided components. The documentation is well done, and after working through the first tutorial you should be able to begin creating your own applications quickly. I’m sure there are other "instant-on" browser-based AJAX application-building tools, but I stopped looking when I found this one. It works without requiring any manual configuration or third-party tool installation. I particularly like the debugging capabilities of the built-in XML Mapping Utility Test Interface, which lets you test your application from inside the development workspace and insert breakpoints in the Web service message flow. This lets you see exactly what the inbound and outbound message contents are.