It's been a fairly busy year for the OneNote test team. One way I can give some "proof" for that statement is by looking at the number of powertoys the test team gave out over the past year. Frankly, there have not been that many, and most of the results here are merely updating addins from OneNote 2007 to OneNote 2010.
And for good measure, I'll add a link to Omer's (excellent) Onetastic powertoy. He's not a tester but since he's on the team I figured I would include a link here.
New for OneNote 2010:
OneCal http://www.omeratay.com/onetastic/ Has a calendar view and image cropping. Super fantastic extension!
Updated from 2007:
Image Importer http://blogs.msdn.com/b/johnguin/archive/2010/12/20/a-bulk-image-importer-powertoy-for-onenote-2010.aspx
Sort Pages http://blogs.msdn.com/b/johnguin/archive/2011/01/24/updating-the-page-sorter-powertoy-for-onenote-2010.aspx
Table Of Contents http://blogs.msdn.com/b/johnguin/archive/2011/04/28/updated-the-onenote-table-of-contents-and-sorter-powertoy.aspx
Section Color Powertoy http://blogs.msdn.com/b/johnguin/archive/2011/07/11/onenote-section-color-powertoy-updated-for-onenote-2010.aspx
Table Sum http://blogs.msdn.com/b/johnguin/archive/2011/09/19/table-sum-toy-updated-for-onenote-2010.aspx
For OneNote 2007:
Tree Control http://blogs.msdn.com/b/johnguin/archive/2011/03/07/a-treeview-control-for-onenote-2007.aspx
This has been replaced in functionality with the Quick Filing Dialog in OneNote 2010.
If you want to write your own addins, here are some links to get you started.
Visual Studio (the Express editions are free) http://www.microsoft.com/visualstudio/en-us/products/2010-editions/visual-csharp-express
The OneNote 2010 Developer Center: http://msdn.microsoft.com/en-us/office/aa905452 Has links to the reference materials and lots of code samples (this is new!).
And one of our external users has written a good step by step guide you may want to study.
Malte Ahren's blog - he tells how to create Ribbon based addins:
Questions, comments, concerns and criticisms always welcome,