Oxygen
Oxygen is a commercial cross-platform XML editor. It claims to be "the only XML tool that supports all the XML schema languages", having support for XHTML, XSLT, and TEI. It features both an XML Editor as well as XML authoring tools, and can also support the creation of XML schemas.

Figure 1: XML grid editor from Oxygen (Click image to enlarge)
Implementing Oxygen
Oxygen is available for all major operating systems (Windows, Mac, and Linux) for purchase or as a free trial. Oxygen can either be used as a standalone tool or as an Eclipse plug-in.
In our implementation, Oxygen was installed on a virtual machine running MS Windows Server 2008R2 on the project server. An RDP file of Oxygen was created, which students then installed on their own computer in order to access the software. Separate RDP files were created for Windows and Mac systems.
Specific minimum installation requirements and client-side installation instructions (for virtualized software only) can be found under the Minimum Installation Requirements and Client-Side Installation sections of the website.