"Ilya created the OneNote OM Spy tool back in 2007. Here it is updated for OneNote 2010. It works much like the original, so I don’t think I need to post a new image or anything like that. There is one change - you can choose which schema you want to use with the dropdown along the top of the window:
As usual, the download link is below my signature at the end of this entry. And the documentation for the schema for 2010 is here.
Since I was talking about Office stuff today, Open XML Opens Office Document Metadata (without Office), also mentioning this seems like a good fit. There's no source available, but still if you're learning the OneNote API or need to spelunk OneNote 2007/2010 Notebooks, this looks like a must have tool.