MOSO Xmplify is a powerful new XML editor for Mac OS X.
Fully XML-aware editing environment
Xmplify analyses your XML as you type, and maintains a rich set of meta-data so it can provide intelligent editing aids and suggestions according to your document's content and the current editing context.
Xmplify provides an Outline view of your document that shows its structure, updated in real-time.
Auto-complete XML based on its DTD or W3C Schema
Xmplify uses its knowlege of your document's current structure and its definition (DTD or W3C Schema) to provide intelligent auto-completion suggestions.
Automatic Schema derivation
If your document doesn't specify a DTD or W3C Schema Xmplify automatically derives one and seamlessly uses the derived schema for providing intelligent auto-completion suggestions. Xmplify updates the derived schema as you edit your document, so it can always offer accurate suggestions.
Automatic document verification
If your document specifies a DTD or W3C Schema, Xmplify automatically uses that to verify your document's content. The verification results are automatically kept up to date as your edit your document.
Xmplify makes applying XSLTs and working with their transformations easy, and with Xmplify's Preview window you can easily check the output of XSLs that produce HTML.
Powerful searching by both XPath and Regular Expressions
And loads more! Full details are at http://xmplifyapp.com.