I sometimes get asked if there is a "viewer" available for OneNote files. There are viewers available for Word, Excel and PowerPoint (among others) so this is a natural question.
The answer for OneNote is yes: it is called the trial version. You download it here for free, and it lets anyone view and search and print OneNote files with perfect fidelity. Oh, it also lets you *edit* and create such files for the first 60days. And you can convert this viewer into the full product by purchasing a license and activating it with the code you get when you purchase (no additional download required).
As I write this, the viewer (trial) is only available for residents of the US, Canada, France, Germany and Japan (thanks for the update Patrick!), but it will be available much more broadly shortly (as it gets rolled out for markets where using a credit card to purchase online is not as feasible). It's also a pretty hefty download (195MB) since it contains the entire product but with broadband it's just a few extra minutes.
Ok, now you might be saying: Chris, don't be smarmy. A trial is not actually a viewer. The difference might be a smaller download, no limitation on which markets can access it, and um, something else I suppose - oh yeah, cross platform. :-)
Many people asking for a viewer are concerned that others cannot read files they send from OneNote. We anticipated that, which is why there are so many options for output in other formats such as Word, PDF, and HTML. And it is also why when you use the email function (with Outlook) we include HTML in the message body so any recipient can view the notes. (a full OneNote format attachment is included for colleagues who use OneNote)
Once in awhile someone asks why there is even a separate OneNote file format at all. After all, if OneNote used some other format there'd be no need for a viewer. Suffice it to say many of the slick things we do with auto-save, multi-user, remote sync of internet-based notes and so on require it, not to mention many other features that are better and more robust as a result. Other file formats are optimized for something other than what we need, be it feature set of some unrelated product, human readability, backwards compatibility with some other older product, etc. We actually have a lot of innovation baked into our format that helps OneNote be what it is.
So, why isn't there a viewer for OneNote other than the trial? The answer is that making a special viewer is non-trivial work, especially testing (and forget cross-platform - that is nearly a whole new product). OneNote is a small team, and every bit of work we do on a viewer means we don't get to work on something else that many people have begged us to add (as I wrote nearly three years ago). The trial covers most people's viewer needs (and then some) so the few extra % of users the viewer would help lose out to the people who are ecstatic that we added other much-requested features to the product instead.
(Update 7/20/07: David Tse from the OneNote team has made a PowerToy that publishes an HTML view of a notebook packaged as an MHT file. It is designed for use with Sharepoint but can be used elsewhere. Give it a try)