My girlfriend just emailed me.  She mentioned that someone at her work saw her using OneNote, and asked her why she liked it.  And she wasn't able to give him a very convincing argument (she claimed that was because "she's probably not using it to it's full potential").  As this was the fourth time this had happened to her, she wanted to know where the one page summary of why someone should use OneNote; the top 10 things it does that would help someone the most.

Here was my list:

• OneNote allows me to put all of my little bits of information into a single place, organize them how I like, and always be able to find them instantly.
• There’s a ton of information that doesn’t naturally have a good home otherwise.  Like the URLs I find when researching a topic, or the notes that I take during meetings.  Sure, I could use text files and notepad or post-it notes; but I’d have to create my own method for filing them into folders, navigate to the correct file to open them when I want to read them again, make sure I remember to hit Save (and give it a filename) before my laptop battery runs out and I lose my content, etc.  It really doesn’t scale when you have a lot of data.
• It’s page surface allows me to outline, brainstorm, and collect rich forms of data better than any other tool out there.
• Specifically, the ability to click anywhere on the page and just drag-drop any line of text to anywhere else on the page means that I can use this for random brainstorming and when writing out document outlines/drafts.  Things that don’t have linear or well-known structures. 
• Plus, there’s a million features built into the application that allows me to embed non-textual forms of information.  So I can use screen clippings (via the Windows + S key) to take a picture of something currently on my screen, or embed a full document via the included OneNote Printer or the Insert Menu, and then annotate on top of that information.  And I can find it again, since we’ll OCR the text within the pictures.
• Even without a tablet PC, the drawing shapes and click anywhere to type means that I can create simple diagrams without having to load up Visio.  With a tablet, I can draw directly on a page, and use a pen when I’m in a meeting where typing may be viewed as distracting.
• It works with audio as well.  We record the audio for all of our spec reviews using the built-in laptop microphone.  Any notes typed during the meeting will be synchronized into the audio timeline for later review.  And OneNote will search the speech in the audio file as well.
• It’s really good at capturing information quickly. 
• Sometimes I need to get information written down as quickly as possible.  I don’t want to worry about making space in my word document, I can just click anywhere on the page and type. 
• Ditto for inserting tables.  Just hit tab! 
• I can launch a side-note window (which is a lot like a post-it note) from the system tray and grab down that phone number that someone just spouted off while I’m on the phone.
• I can paste web content from a web page and it automatically includes the URL the content came from.  Huge time-saver.
• I can apply metadata (flags) to my information or create Outlook Task items “in situ” along with the rest of the context that gives that task meaning.
• I’m no longer restricted to keeping a single task list in Outlook.  When I’m in a meeting, or estimating a feature in OneNote I can tag a line as an Outlook Task, and it’ll create an Outlook Task for me, which is automatically kept in sync as I mark it completed, etc.  As a result, all of my ToDo items can live in the place where they’re most appropriate (like in the middle of my meeting notes, or in my shared notebook with you on a page of house projects) and yet have them rolled up appropriately in either OneNote or Outlook.
• Outlook Integration, Outlook Integration, Outlook Integration.
• In addition to task sync’ing, I find that there’s a ton of information that gets sent to me in email, which should live in OneNote instead.  (As email is more of a dynamic source of changing data, vs. an authored knowledge base.)  I can send an email to OneNote directly from Outlook 2007 via a single toolbar button click.  For someone who tries to keep their inbox nearly empty, being able to store messages like “how to access the internal newsgroups” (for instance) in a Notebook feels much cleaner than keeping them in my inbox.
• In addition, I can also take notes about meetings (and have it find my previous meeting notes for a recurring meeting) or keep information about people from my Contact List / GAL in OneNote directly from the Outlook meeting and contact windows.  The link between the two stays present regardless of how that gets filed in my Notebooks.
• My stuff is available everywhere.
• I can’t emphasize how much this rocks.  My OneNote notebook is available at work, at home, on my phone (using OneNote mobile) and on my laptop.  All I did was point OneNote at a file share or Sharepoint Site, and OneNote takes care of the rest.  Plus, it synchronizes embedded documents as well, so I don’t have to use Sharepoint to upload a document or email it to myself.  I just drag-drop it right onto the OneNote page, and voila it’s everywhere I need it!  No sync’ing, no file locking, nothing.  It just works.
• Moreover, it works when I’m offline.  Even those embedded documents… when I pick up my laptop and go to a conference room in another building, I can still keep typing, regardless of whether or not I’ve got wireless.  Go on vacation to the beach, and make changes to my notebook.  Whenever it comes online, it all merges back in without any user interaction.
• It allows me to collaborate with others.
• Word track-changes?  Sharepoint edit locks?  Yuck.  OneNote is a breeze by comparison.  Think of it like a Wiki on crack.  Everyone just opens up the same Notebook (or Section or page) and just types away.  It’s magic.
• For those without OneNote, I can create PDFs of my pages, or send a page as an email with a single click.  The person on the other end of that email doesn’t even need OneNote to view my stuff.
• I can store sensitive information and password protect it.
• I generally use this for my personal notebook, but I find it invaluable to store my Credit Card numbers, Bank Account Information, Website Passwords, Frequent Flyer accounts, etc. all in a section that I then password protect.  Because the bits stored on disk are encrypted, I can access that file from a server and not worry about the security of the server, across the network, etc.
• I can automate repetitive things.
• I keep a work journal, and find that it’s really convenient to create a stationary (templates) page which is applied automatically to all new pages created in my Journal section.  It’s such a simple idea, but saves me a ton of time.
• Not to mention all the cool add-ins that power-users have created that extends the functionality of OneNote. 

Hmm... that's only 9, meaning I've got room for at least one more.  

What are the reasons why you use OneNote?