The folks at SyncRO Soft have released the 6.2 version of oXygen, my personal favorite XML/XSL/Schema editor! Details on the new features are available at: http://www.oxygenxml.com/index.html#new-version.
What I like about oXygen is it's standards support (especially RelaxNG!) and it's highly configurable parsing features. It has Xerces, Xalan, Saxon, FOP , Jing, Trang and xsltproc built in.
It's extremely powerful, including support for XML 1.0, namespaces, DTD, XML Schema 1.0, Relax NG, XSLT 1.0 and 2.0, XPath 1.0, XSL-FO, XQuery 1.0, SVG 1.1, WSDL, CSS, Unicode 3.2, DocBook 4.3, TEI and Schematron!
My other favorite feature: it's multi-platform. I have personally run it in Solaris, Mac OS X, and Windoze.
This editor isn't as user-friendly for the common author as Arbortext Epic, but for XML hacks, I'd say oXygen has all the other editors beat - hands down.
Try it for yourself. There's a free 30-day trial available on the site. I'd be interested in your comments on oXygen and other editors, too.
See also: XML