Oxygen XML Editor
From the product website (accessed 2016-04-20):
oXygen XML Editor is the complete XML editing solution, both for XML developers and content authors. It provides must have tools for XML editing, covering most XML standards and technologies. oXygen XML Editor includes all the features of oXygen XML Developer and Author.
Oxygen is bundled with schemas, ODD, and templates for TEI and several other common XML schemas. Many Digital Humanities workshops that involve TEI or EpiDoc training recommend the use of Oxygen (despite the habitual push toward open source tools in these communities), on the grounds that it is very easy to use, has almost all functionalities an XML coder or developer might need, is relatively affordable by software standards, and that the developers at SyncroSoft have engaged with the DH community and are reponsive to feature requests.
An academic license for Oxygen costs $99 ($119 with one year support) as of 2019-06.