I was hooked on OneNote the first time I saw it beta before Office 2003 was released. When I was at Microsoft, I had a couple of opportunities to meet Chris Pratley and he always showed me some amazing feature that I never knew about.
There are a few features within OneNote 2007 that I find the most useful. The first is the Screen Clipping tool that is built it. I write technical articles and documentation often so having a easy to use screen clip is very valuable. I've used many third-party screen capture software, but the one built into OneNote fits my needs perfectly. The nicest feature is that works outside of OneNote, not just while you're in the application. The Windows+S hotkey frames the screen and allows you select the input that you want to capture. The image below is captured right after I will write this line…
Screen clipping taken: 12/17/2006, 1:05 PM
The second feature is OneNote 2007's integration with Word's blog posting feature. I always begin writing in OneNote and typically transfer what I've written to Word or copy-paste into Community Server to post a blog entry. Now, I can write in OneNote and publish it directly to my blog (note: Word opens up to review before publishing). This little time saver may provoke me to post more often.
The last feature is the build in calculator. Open a note, type something like "Pi * 3 / 2 =" and hit enter and you'll get Pi * 3 / 2 = 4.71238898038469. When I first saw this I thought it was a neat feature, but since know it was there, I use it at least weekly.
Hope you find this useful and thanks to Chris and the OneNote team.