oXygen XML editor
A cross-platform XML editor that may be used to create and validate XML documents and associated schema. It fully supports XSL (both XSLT and FO), DTD, Schema (Relax RNG and W3C), Database, XQuery and CSS. OXygen XML Editor works with all XML-based technologies, including XML databases, XProc pipelines, and web services and comes with ready-to-use DITA, DocBook, TEI, and XHTML support. Frequently updated and supported, and with a very large set of features, this software tool has proved popular with digital humanists.
The Versioning Machine displays multiple versions of text encoded according to TEI Guidelines and allows for comparisons of annotation and introductory materials. This is a text editor and allows editors "to immediately see the consequences of their editorial decisions." This tool does not appear to have been updated since 2011.
Digress.it is a WordPress plug-in that allows paragraph-level commenting in the margins of a text. Use digress.it as a collaboration tool for blog-style comment threads off individual paragraphs. Host your project on digress.it or download it for use with your own WordPress installation.