HarBox is the first (and only) Box.com client for Nokia N9. It is currently in development stage, but a beta version can already be downloaded and tested at Nokia Developer Projects website.
To install it, here's a direct link to the.deb installation file. Download it, copy to your N9 (bluetooth or USB) and run with an N9 file manager (e.g. FileBox) to install. Remember to switch on "Allow installations from non-Store sources" in Settings -> Applications -> Installatons. Read more on installing non-Store applications here.
The app already looks fine (though simple) and works a treat. I'm keeping my fingers crossed for Le Bien Cuong, the guy responsible for the HarBox project.