oXygen XML Editor combines an easy to use visual editor with a powerful code based editor thus covering the needs of both authors that do not want to see the tags and XML gurus that want to have full control over their markup.  

DITA support comes built in consisting of

XMLmind XML Editor allows to edit large, complex, modular, XML documents. It makes it easy mastering XML vocabularies such as DocBook or DITA. (More info.)

As you can see it in the screen shot below, XMLmind XML Editor is not a tool for programmers. Its users are generally technical writers.

oXygen is a cross-platform XML Editor. Besides the top-of-the-line XML source editing support, it features a CSS driven WYSIWYG XML editing mode allowing the content authors to visually edit DITA documents. oXygen features include out-of-the-box integration with the DITA Open Toolkit and can be easily customized to match your DITA editing and publishing needs.

Astoria On-Demand is a fully-web hosted and accessible xml content management solution for content publishing to the DITA standard.  Astoria manages and automates the creation, review, assembly and publishing of technical content, such as technical product information, where content is complex, is re-used, shared and/or re-purposed across multiple langugages and delivery channels.

Syntext Serna is an easy-to-use WYSIWYG XML editor that works on Windows, Linux, Mac OS X, and Sun Solaris/SPARC. Serna supports DITA, Docbook and other popular XML document types out-of-the-box. S1000D support is also available.

Serna looks and works like a conventional word processor, yet it gives you great flexibility and a full range of means of authoring structured XML content. Serna lets you work with documents composed of various reusable components and viewed as a transparent integrated whole.

DITA Support