XML Copy Editor is a free, open source validating XML editor. It is a fast, no frills XML editor that provides users with the essential features needed to edit, format, and validate XML documents. It features a customizable XML tree view, an integrated XML Schema validator, a customizable syntax highlighting editor, a customizable code completion feature, and an integrated HTML preview. It also provides users with the ability to perform search and replace operations on XML documents, and can be extended with additional validating parsers. XML Copy Editor is a great tool for developers and designers who need to quickly edit and validate XML documents, and is available for Windows, Mac OS X, and Linux.
Notepad ++ is only a text editor, it does not seem to offer any XML processing functionality (e.g. validation, etc.).