Forest is a Web-based IDE for Web applications. It's in the alpha stage, but you can use it to create your XML files already. For now, it has four frames: the "Add Component" frame which allows you to add a new component to your page, the "Object Inspector" which allows you to add/edit properties and delete components, the "Preview" which will show how your XML is looking, and the "Component Tree" frame that show the hierarchy of components in your page.
Release Notes: A Debian package has been created.
Release Notes: A resize link was added for the Component Tree, which shows and hides the frame. The class list for new components is now grouped with a Optgroup, and the component tree now has links to open the a component's properties.
Release Notes: A MaskedInput component has been added.
Release Notes: The object inspector no longer breaks when updating its properties immediately after the add command. There is a new PageNavigator component.
Release Notes: A new DateTimeField component, and many bugfixes.