There are newly published apis for accessing OneNote that you can get started building apps for today. http://msdn.microsoft.com/en-us/library/office/dn575420(v=office.15).aspx
WPCentral has good coverage on the new OneNote features that were recently announced: Microsoft introduces new OneNote APIs, free desktop apps for Mac and Windows
Personally the one I find most interesting is the [email protected] feature where you can simply send an email and have the contents appear within OneNote (yeh I know Evernote’s had that for years). What I love is the simplicity – it detects which Microsoft account based on which email address you send it from. All you have to do is remember “[email protected].com”. To get started:
* Go to http://www.onenote.com/apps
* Click through to “Email to OneNote”
* Enrol your email address.
* Start sending emails to [email protected]