XML is a powerful and flexible way to encode data, but designing XML "schema" (the rules by which data is encoded) is often difficult. Verifying that your XML documents conform to the schema can be difficult, as well. XMLFox XML Editor is a nice freeware program that greatly simplifies these tasks.
For the uninitiated, XML is a distant cousin to HTML, the language used to describe Web pages. XML is defined via "schemas" which describe which tags are allowed, which order they may go in, and what values they may have. XMLFox is a very cunning (like a fox!) program which will allow you to begin entering essentially arbitrary XML, and will work out the schema from it. In short, you create your XML file and you get a working schema, which you can then save and reuse.
In addition to this, XMLFox has the basic features one expects from an editor. It detects missing or malformed tags (for example, starting a tag with <Item> and closing it with </item>; XML's case-sensitive tags often confuse those familiar with only HTML). XMLFox has colored syntax highlighting, and shows your raw XML in three formats: grid, tree, and script. In the left hand pane, you see your ever-evolving schema.
XMLFox lacks a lot of high-end, complex, editing features found in some editors, such as macros, regular expression parsing, and so on--but it's quick, clean, and easy to use. It also allows for the editing of only one XML document at a time, which is the only serious impediment to productivity I have found. If XML is a sideline rather than a full time job, this simplicity and cleanness handily trumps a lot of complex generic text editing features you will likely rarely need. As a free editor which does its one job well, XMLFox worth looking into.