You're probably seeking an XML editor with features like validation, syntax checking, and possibly even GUI-based DTD, XSLT, XPATH etc? I don't know of any free ones, though several good commercial packages have trial versions.

In the free zone I'd recommend Notepad++. It's not an XML editor per se, but it does a nice job of colorizing and highlighting XML, and comes with a bunch of plugins for checking well-formedness, finding nodes, and more.

moderator's note: please do not evangelize commercial products. But free ones are OK to mention by name as long as they're actually 100% free.

Suprisingly, even I'd recommended Notepad++.
For its quick, easy, fast and useful for all purpose but some users here used to WYSIWUG, not liking it, and they insist on former one.

Well, you may edit editor name, in my previous post; I'd tried but unable to edit.

try TED Notepad