Microsoft OneNote is a part of Microsoft Office that I've never heard of. Apparently it's fairly commonly used amongst Windows users; I guess it came after I left Windows.
Anyway, it's a note taking app that's also part of Microsoft's cloud platform (Google Docs competitor). They've built a plugin that makes it pretty straightforward to take Microsoft OneNote docs and publish them to WordPress.
This may be boring news to many of you, who like publishing in WordPress, but I'd bet your clients (who are probably more often Windows users than Mac users) would appreciate it.