Format HTML document
I like the feature in Visual Studio for formatting an HTML document (CTRL+K, CTRL+D).
Is there a plugin with the same functionality in any other (free) editors, e.g. Notepad++?
If you mean colorizing and formatting from "Format", Notpad++ has HTML tidying feature via Tidy2 plugin. Install the plugin with the Plugin Manager, and a new menu Tidy2 item will appears under the Plugins, and from there you can formatting HTML.
Also Netbeans IDE has formatting(ALT+F) feature for many languages (including PHP, HTML, CSS and JS).