One of the features I just *love* about OneNote 2007 is the "shared notebook" feature.   You can create a notebook that co-workers, friends, etc can also edit at the same time.  OneNote was already an amazing app, but this makes it all that much better.

Notice the SharePoint parts I circled in the wizard above?  That's because the UNC path approach to a shared notebook is drop-dead easy inside your corporate firewall, but it's not as simple when you're trying to share a notebook with others outside the firewall, like vendors, contractors, off-site employees, etc.

This is one of the times when including SharePoint in the box with TFS gets us a cool scenario "for free" (mmm synergistic love).  You can use your SharePoint site for your team project, create a shared notebook on it, and whoever can access your TFS / WSS can now work with your new shared notebook!  We have lots of customers that expose TFS out on the internet (through an ISA reverse proxy, or putting TFS into a DMZ, or even just sticking it out on a machine with a public IP), and now you can send these onenote:// links to your friends/coworkers/etc (just right-click on the notebook, or a section, or a page while you're working with it to select such a hyperlink). 

On a related note, configuring TFS to work for both internal and external clients in 1.0 wasn't an easy thing to do, and we're working on making that better in future versions.