I've just finished the first version of a Visual Studio Debugger Visualizer for the Html Agility Pack. It allows to browse the DOM / Nodes while debugging. XPath expressions can be evaluated during the debug session.
Here is the project:
http://www.crawler-lib.net/visual-studio-debugger-visualizers
Here is the project:
http://www.crawler-lib.net/visual-studio-debugger-visualizers