This is a free xml integrated development environment.

Features

• Extensible
• Support for external tools, such as XSL processors, XML formatters, and web browsers
• Color-coded XML editing
• Ability to open files via URL
• Checks well-formedness and optionally validates XML documents
• Extensible library of Code Bits, which are cataloged text fragments that can be inserted into any document and shared with other users
• XML Navigator - Ctrl+3
• Spell checker (requires MS Word)
• Auto workspace: automatically remembers the pairs of xml/xsl files used
• Custom file extensions for xml, xsl, and dtd files
• Various text-entry short cuts -- see Help/Keyboard Map for a complete list.
• XPATH Support
• Extraction of XML tags and XPATHs - Ctrl+2
• XPATH testing and generation
• Stylesheet testing
• Displays generated HTML in browser window (integrated or external)
• Support for multilevel XSL includes and imports
• XSLT Run-Time Parameters
• Document formatters and reports
• Conversion of HTML to XHTML via tidy
• MS XSL to XSLT converter
• Peer-to-peer features
• Sharing of Code-Bits and Web Favorites
• More coming soon
• Integrated web browsing
• Extensible library of XML and XSLT related resources on the net

I have to say this is a brilliant piece of softare. There isn't much around to compare with it.