XML Notepad is a small free application from Microsoft that aims to provide a small environment for editing XML files quickly. XML is a standardized data exchange format, commonly used in web environments but with more and more uses each day.
XML Notepad has a simple yet powerful user interface, where you can browse through your XML file and edit it. Drag and drop, and IntelliSense options facilitate most of the tedious work that supposes the editing of the plain text.
Over the left side you can see your XML file as a tree structure, notice the colored dots (red, green, purple) indicating the node type (attribute, comment or processing instruction), and other ones like folders (elements). You can select any node and switch between one type or another.
It's really easy to work with complex files thanks to their unique "nudge" operations: select a nudge up/down (to move a node before/after its siblings or parent), and nudge left/right (to move a node out of its parent, at the same level).
There are many other interesting features like the "Compare XML files" that uses a Diff engine to achieve the best results.