I found this on a Google search:
http://www.xmlcooktop.com/

Haven't tried it though, but it's free, so it couldn't hurt.

Altova Enterprise XML Suite are simply unbeatable! :)

http://www.altova.com

I second the Altova products. Been using it for years and it easily integrates into many development environments (visual studio, etc).

this software is ridiculously expensive. I downloaded a trial and was sad I could not afford the thing:

Tibco's TurboXML
http://www.tibco.com/software/business_integration/turboxml.jsp

they have other neat products for XML. Although they are a ESB shop.

nxml-mode is a fantastic XML editing extention for Emacs:

http://www.emacswiki.org/cgi-bin/wiki/NxmlMode

I really can't imagine editing XML with anything else.

Jedit: http://www.jedit.org is now my editor of choice -- if you are willing to spend time customizing it. It has an extensible architecture, including xml plugins to pretty print, run XSLs, evaluate XPaths, etc. Takes a while to get used to, but you can customize most aspects of it (including the various keyboard shortcuts)

I use XML Spy too -- the paid for version has an XSL debugger, but I have found myself only ever using it once or twice, in over 4 years of having it. Its pretty print it good, and xpath evaluator is handy too. However, the editor itself is awful in my opinion as it slows down if you have too a few files open for a long time, its search replace is slow and limited in features, etc!

Hope that helps

I?m a big fan of the oXygen (http://oxygenxml.com) plugin for Eclipse (http://eclipse.org). It integrates well into the fee, flexible, cross-platform Eclipse IDE. It does validation/well-formed checking, XSLT and even comes with a visual schema editor and CSS and JavaScript editors.

I don't have a need for one often, but I've checked out a bunch of the freeware ones and XMLPad 2.0 was the best I came across...

http://www.wmhelp.com/xmlpad2.htm

Uh.... I've always just used my favorite text editor. Altova's XML spy was a little too bloated for me.

CookTop (already mentioned in the first comment) works great with delicious syntax highlighting, auto-indentation and nice evaluation with preview as pure text or html. Really a nice piece of freeware.

I like Cladonia Exchanger XML Lite, see: http://www.freexmleditor.com/

I prefer EditiX because it is cross-platform and is a very good XML editor compared to other. It displays syntax error on the fly and shows always the current XPath location. It is ergonomic too.

http://www.editix.com

So everyone has their own favorite?

Well, then i can add my favorite too: xmlBlueprint (www.xmlblueprint.com). Supports DTD, RelaxNG, XMLSchema and XSLT. It supports many different parsers (Xerces, etc.). The "intellisense" is quite good, and context sensitive, much better than XMLSpy! And, xmlBlueprint is cheaper (about $40) than many other XML editor.

This was a big disappointment. Most people simply said "oh, I use XYZ and like it". What we need are comparisons by people who have used a variety of editors over an extended period of time.

I use a free one call editx at http://free.editix.com

This is enough for me, have a look

I have used two editors, one is Oxygen XML which is very popular (but someone has already mentioned this one). The second editor that I have used is Liquid XML editor and this is my favourite. It is £65 (around $105) for the basic version so it is in a similar price bracket to the basic version of Oxygen XML and it supports XSLT, XQuery as well as allowing code generation with an XML Schema. My only quarrel with it is that it does not support RelaxNG.

I don't want to pay for a simple text editor, try this xml editor editiX free version