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.
